Now user can filter events with types. The API is also extensible.
It might make sense for the proxy to filter out events based on
more expensive/customized filter.
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
Instead of holding the store lock while doing a lot of work like when syncung
unsynced watchers, the work from a blocked synced notify can be reused and
dispatched without holding the store lock for long.
