Migrate MonitorUpdatingPersister to an async + async-sync wrapper

As we've done with several other structs, this adds an async
variant of `MonitorUpdatingPersister` and adds an async-sync
wrapper for those using `KVStoreSync`. Unlike with other structs,
we leave `MonitorUpdatingPersister` as the sync variant and make
the new async logic a `MonitorUpdatingPersisterAsync` as the async
monitor updating flow is still considered beta.

This does not yet expose the async monitor updating logic anywhere,
as doing a standard `Persist` async variant would not work for
ensuring the `ChannelManager` and `ChainMonitor` don't block on
async writes or suddenly require a runtime.

Author: TheBlueMatt