Channel Manager Persist-Required Checker

[TheBlueMatt]

We need to improve the await_persistable_update API in ChannelManager with at least a few things -

• It should be possible to detect when if a new update has come in while we were'nt awaiting - eg the comment here notes the obvious pattern used actually may miss persistence events - Call peer_manager.process_events() in BackgroundProcessor #864 (comment)
• We should avoid generating persistence events for events that obviously don't need it - eg we had a user complain about duplicate block/transaction connection events generating persistence, but some other candidates include timer_chan_freshness_every_min (if it didn't do anything).

[TheBlueMatt]

I believe this is largely addressed. If we have any more cases of excess updates we can open new issues for them.