This allows for watchers to be created concurrently
without needing potentially complex and latency-adding
queuing on the client.
Signed-off-by: Gyuho Lee <gyuhox@gmail.com>
If Close() is called before Cancel()'s cancel() completes, the
watch channel will be closed while the watch is still in the
synced list. If there's an event, etcd will try to write to a
closed channel. Instead, remove the watch from the bookkeeping
structures only after cancel completes, so Close() will always
call it.
Fixes#8443
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.
