Make MonitorUpdatingPersister change persist type based on size

[TheBlueMatt]

One thing that smaller LDK clients might want is to use MonitorUpdatingPersister in many cases, but not if the ChannelMonitor is only, say, a few KiB when serialized. This avoids the overhead of compaction later if the actual monitor being persisted is tiny anyway.

I think this should be quite easy to do so tagging as a good first issue.

[0xZaddyy]

can i work on this sir?

[TheBlueMatt]

Sure!

[joostjager]

This avoids the overhead of compaction later if the actual monitor being persisted is tiny anyway.

Is this significant if the channel monitor is small anyway?

[TheBlueMatt]

If the storage layer is remote and high latency I think it could be? I guess in theory if we're really careful about the lookup logic (and the KVStore is async) we could do the loading in parallel (though its still a few more requests) but we'd still have to wait on compaction (without a background-compaction task running, which would require a generic spawn method...). Seems much easier to just check the length and skip some code for the common case :)

[joostjager]

If I understand it correctly, this is special casing for channels at the beginning of their life and when they grow, the node will switch over to persisting updates and compaction anyway?

To me it isn't quite clear if this is really worth it. If the hit is significant, it is also a problem when the channel has outgrown the 'small' stage anyway?