In case syncWatchersLoop() starts before Restore() is called,
watchers already added by that moment are moved to s.synced by the loop.
However, there is a broken logic that moves watchers from s.synced
to s.uncyned without setting keyWatchers of the watcherGroup.
Eventually syncWatchers() fails to pickup those watchers from s.unsynced
and no events are sent to the watchers, because newWatcherBatch() called
in the function uses wg.watcherSetByKey() internally that requires
a proper keyWatchers value.
Existing dbSize shows physically allocated DB size and the backend
(boltdb) won't shrink it after a compaction until a user runs the defrag command.
The new dbSizeInUse shows the DB size that excludes free pages created
by compactions so that users can see the actual DB usage. dbSize >=
dbSizeInUse is always true.
Note that dbSizeInUse shows a page-based size and not byte level usage.
Xiang