Monitor updating persister

[domZippilli]

Monitor-updating KV store persister

Adds a new MonitorUpdatingPersister struct that customers may choose to use, so that new events on a channel do not serialize the entire monitor to storage.

How it works

• Stores monitors in monitors/
• Stores updates in monitor_updates/
• When the "update persisted monitor" call comes in, either:
  • Rewrites the monitor, cleaning up old updates at this time, or
  • Adds an update file
• At init when it deserializes monitors, it:
  • Reads each monitor and deserializes it
  • Reads each update for that monitor, deserializes it, and applies it if it has an update_id > the monitor's, or is the closed channel update constant.

Migration Path

No migration should be necessary! It should "just work" since the full monitors are stored the same way. When this persister runs the first time, it'll deserialize the existing full monitors, and the start writing updates.