Avoid persisting a ChannelManager update after each timer tick

Currently, when a user calls `ChannelManager::timer_tick_occurred`
we always set the persister's update flag to true. This results in
a ChannelManager persistence after each timer tick, even when
nothing happened.

Instead, we add a new flag to `PersistenceNotifierGuard` to
indicate if we should skip setting the update flag.

Author: TheBlueMatt