Process background events when taking the total_consistency_lock

When we generated a `ChannelMonitorUpdate` during `ChannelManager`
deserialization, we must ensure that it gets processed before any
other `ChannelMonitorUpdate`s. The obvious hook for this is when
taking the `total_consistency_lock`, which makes it unlikely we'll
regress by forgetting this.

Here we add that call in the `PersistenceNotifierGuard`, with a
test-only atomic bool to test that this criteria is met.

Author: TheBlueMatt