Current tests don't normally trigger the watch victim path because the
constants are too large; set the constants to small values and hammer
the store to cause watch delivery delays.
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.
Anthony Romano