We used to wait on ChannelMonitor persistence to avoid
duplicate payment events. But this can still happen in cases where
ChannelMonitor handed the event to ChannelManager and we did not persist
ChannelManager after event handling.
It is expected to receive payment duplicate events and clients should handle these
events in an idempotent manner. Removing this hold-up of events simplifies
the logic and makes it easier to not persist ChannelMonitors on every block connect.

We only used to store last_chain_persist_height to release
events held for more than LATENCY_GRACE_PERIOD_BLOCKS due to
pending monitor update with UpdateOrigin::ChainSync. Since we no
longer pause events for ChainSync persistence, we no longer need to
store last_chain_persist_height.

We no longer need to track them since we no longer hold events for
pending MonitorUpdates resulting from ChainSync.

MonitorUpdateId was an opaque abstraction for id's generated by
UpdateOrigin:Offchain and UpdateOrigin::ChainSync monitor updates.
It was mainly needed to map calls made to
ChainMonitor::channel_monitor_updated. We no longer track
UpdateOrigin::ChainSync MonitorUpdates and can directly use
ChannelMonitor::get_latest_update_id() for tracking
UpdateOrigin::Offchain monitor updates.

It was used earlier for generating unique MonitorUpdateId for
UpdateOrigin::ChainSync monitor updates.