Substantially optimize reference-row-fetching

Substantially optimize reference-row-fetching

By first fetching the rows we need from a smaller index, we avoid
walking a large index which contained the full `blob_signed`. This
reduces reference-row-fetching from 680 seconds to 152 seconds when
searching today for reference rows against 7 days ago.

Old:

```
ln-gossip=# EXPLAIN ANALYZE SELECT DISTINCT ON (short_channel_id, direction) id, blob_signed, direction
        FROM channel_updates
        WHERE seen < '2023-07-07 00:00:00' AND short_channel_id IN (
            SELECT DISTINCT ON (short_channel_id) short_channel_id
            FROM channel_updates
            WHERE seen >= '2023-07-07 00:00:00'
        )
        ORDER BY short_channel_id ASC, direction ASC, seen DESC;
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Unique  (cost=186279.46..11921204.82 rows=168910 width=161) (actual time=732.365..680504.173 rows=129985 loops=1)
   ->  Merge Join  (cost=186279.46..11632998.93 rows=57641177 width=161) (actual time=732.364..679193.755 rows=31714061 loops=1)
         Merge Cond: (channel_updates.short_channel_id = channel_updates_1.short_channel_id)
         ->  Index Only Scan using channel_updates_scid_dir_seen on channel_updates  (cost=0.56..10718853.69 rows=57641177 width=161) (actual time=0.638..673675.749 rows=57408667 loops=1)
               Index Cond: (seen < '2023-07-07 00:00:00'::timestamp without time zone)
               Heap Fetches: 0
         ->  Unique  (cost=186278.90..192574.84 rows=84455 width=8) (actual time=478.881..750.241 rows=68210 loops=1)
               ->  Sort  (cost=186278.90..189426.87 rows=1259188 width=8) (actual time=478.878..653.035 rows=1452661 loops=1)
                     Sort Key: channel_updates_1.short_channel_id
                     Sort Method: external merge  Disk: 17680kB
                     ->  Index Only Scan using channel_updates_seen_scid on channel_updates channel_updates_1  (cost=0.56..41481.08 rows=1259188 width=8) (actual time=0.885..264.333 rows=1504495 loops=1)
                           Index Cond: (seen >= '2023-07-07 00:00:00'::timestamp without time zone)
                           Heap Fetches: 2273
 Planning Time: 0.164 ms
 JIT:
   Functions: 9
   Options: Inlining true, Optimization true, Expressions true, Deforming true
   Timing: Generation 21.265 ms, Inlining 37.914 ms, Optimization 113.040 ms, Emission 101.901 ms, Total 274.121 ms
 Execution Time: 680601.155 ms
(19 rows)
```

New:

```
ln-gossip=# EXPLAIN ANALYZE SELECT id, direction, blob_signed FROM channel_updates
    WHERE id IN (
        SELECT DISTINCT ON (short_channel_id, direction) id
        FROM channel_updates
        WHERE seen < '2023-07-07 00:00:00'
        ORDER BY short_channel_id ASC, direction ASC, seen DESC
    ) AND short_channel_id IN (
        SELECT DISTINCT ON (short_channel_id) short_channel_id
        FROM channel_updates
        WHERE seen >= '2023-07-07 00:00:00'
    );
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Hash Join  (cost=2942503.92..2943867.77 rows=169870 width=145) (actual time=22862.627..152436.685 rows=130116 loops=1)
   Hash Cond: (channel_updates.short_channel_id = channel_updates_2.short_channel_id)
   ->  Nested Loop  (cost=2738282.26..2739200.18 rows=169870 width=153) (actual time=22141.452..151504.140 rows=393250 loops=1)
         ->  HashAggregate  (cost=2738281.69..2738283.69 rows=200 width=4) (actual time=22139.440..22339.035 rows=393250 loops=1)
               Group Key: channel_updates_1.id
               Batches: 1  Memory Usage: 45089kB
               ->  Result  (cost=0.56..2736158.32 rows=169870 width=21) (actual time=0.102..21984.409 rows=393250 loops=1)
                     ->  Unique  (cost=0.56..2736158.32 rows=169870 width=21) (actual time=0.074..21943.089 rows=393250 loops=1)
                           ->  Index Only Scan using channel_updates_scid_dir_seen_desc_with_id on channel_updates channel_updates_1  (cost=0.56..2448011.03 rows=57629457 width=21) (actual time=0.073..19776.181 rows=57408667 loops=1)
                                 Index Cond: (seen < '2023-07-07 00:00:00'::timestamp without time zone)
                                 Heap Fetches: 0
         ->  Index Only Scan using channel_updates_id_with_scid_dir_blob on channel_updates  (cost=0.56..4.60 rows=1 width=153) (actual time=0.328..0.328 rows=1 loops=393250)
               Index Cond: (id = channel_updates_1.id)
               Heap Fetches: 0
   ->  Hash  (cost=203159.97..203159.97 rows=84935 width=8) (actual time=721.105..721.107 rows=70731 loops=1)
         Buckets: 131072  Batches: 1  Memory Usage: 3787kB
         ->  Unique  (cost=195708.67..202310.62 rows=84935 width=8) (actual time=552.965..713.465 rows=70731 loops=1)
               ->  Sort  (cost=195708.67..199009.65 rows=1320391 width=8) (actual time=552.962..650.323 rows=1537141 loops=1)
                     Sort Key: channel_updates_2.short_channel_id
                     Sort Method: external merge  Disk: 18064kB
                     ->  Index Only Scan using channel_updates_seen_scid on channel_updates channel_updates_2  (cost=0.56..43421.19 rows=1320391 width=8) (actual time=66.736..324.130 rows=1537141 loops=1)
                           Index Cond: (seen >= '2023-07-07 00:00:00'::timestamp without time zone)
                           Heap Fetches: 68
 Planning Time: 0.520 ms
 JIT:
   Functions: 21
   Options: Inlining true, Optimization true, Expressions true, Deforming true
   Timing: Generation 0.643 ms, Inlining 7.055 ms, Optimization 33.167 ms, Emission 25.782 ms, Total 66.648 ms
 Execution Time: 152458.777 ms
(29 rows)
```