Track in-flight `channel_announcement` lookups and avoid duplicates

Track in-flight `channel_announcement` lookups and avoid duplicates

If we receive two `channel_announcement`s for the same channel at
the same time, we shouldn't spawn a second UTXO lookup for an
identical message. This likely isn't too rare - if we start syncing
the graph from two peers at the same time, it isn't unlikely that
we'll end up with the same messages around the same time.

In order to avoid this we keep a hash map of all the pending
`channel_announcement` messages by SCID and simply ignore duplicate
message lookups.