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.
Makes w.cur into w.minrev, the minimum revision for the next update, and
retries cancelation if the watcher isn't found (because it's being processed
by moveVictims).
Fixes: #5459
