Track "steady-state" channel balances in history buckets not live

Track "steady-state" channel balances in history buckets not live

The lower-bound of the scoring history buckets generally never get
used - if we try to send a payment and it fails, we don't learn
a new lower-bound for the liquidity of a channel, and if we
successfully send a payment we only learn a lower-bound that
applied *before* we sent the payment, not after it completed.

If we assume channels have some "steady-state" liquidity, then
tracking our liquidity estimates *after* a payment doesn't really
make sense - we're not super likely to make a second payment across
the same channel immediately (or, if we are, we can use our
un-decayed liquidity estimates for that). By the time we do go to
use the same channel again, we'd assume that its back at its
"steady-state" and the impacts of our payment have been lost.

To combat both of these effects, here we "subtract" the impact of
any just-successful payments from our liquidity estimates prior to
updating the historical buckets.