One really rough thing about how we did onchain fees is that the records update
every time a new event comes in.
The better way to do this is to create new entries for every adjustment,
so that reconciliation between printouts isn't a misery.
We add a timestamp and `update_count` to these records, so you can
roughly order them now (and have a good idea of the last time an event
that updated an onchain_fee occurred).
When the node starts up, it records missing/updated account balances
to the 'channel' events... which is kinda fucked for wallet + external
events now that i think about it but these are all treated the same
anyway so it's fine.
This is the magic piece that lets your bookkeeping data startup ok on an
already running/established node.
clightning doesn't give us any info about onchain fees (how could it?
it only knows about utxo object levels, and doesn't keep track of
how/when those are all related)
Instead, we keep running totals of the onchain fees for utxos. This
implements the master method for accounting for them, plus includes
tests to account for channel opens (across two accounts) as well as a
htlc-tx channel close.
Missing: we don't currently emit an event from cln for `withdraw`
initiated removal of funds, so the accounting for wallet -> external
funds is a bit janky. We don't account for the fees on these
transactions since we don't have the resulting 'external' event to
register them against!
