Do not wait in PersistenceNotifier when the persist flag is set
When we had a event which caused us to set the persist flag in a
PersistenceNotifier in between wait calls, we will still wait,
potentially not persisting a ChannelManager when we should.
Worse, for wait_timeout, this caused us to always wait up to the
timeout, but then always return true that a persistence is needed.
Instead, we simply check the persist flag before waiting, returning
immediately if it is set.