Use `ChannelMonitorUpdate`s in `FilesystemPersister`

[TheBlueMatt]

I thought we had an issue for this, but I guess not. We should really use the ChannelMonitorUpdates in the FilesystemPersister instead of always persisting the full ChannelMonitor.

Because we're moving towards the logic not being encapsulated in the FilesystemPersister in #1417 we'll have to do a bunch of work there - probably we'll want to move the read_channelMonitors thing into lightning using a get method added to the KVStorePersister trait, with the ability to roll forward the monitors using updates and then remove the updates after persisting the new monitor.