Add MonitorUpdatingPersister

MonitorUpdatingPersister is an implementation of Persister that stores
ChannelMonitorUpdates separately from ChannelMonitors. Its RFC is
in #2545, at https://github.com/orgs/lightningdevkit/discussions/2545.

Co-Authored-By: Elias Rohrer <[email protected]>

Author: domZippilli