Basic support for lease operations like create and revoke.
We still need to:
1. attach keys to leases in KV implmentation if lease field is set
2. leader periodically removes expired leases
3. leader serves keepAlive requests and follower forwards keepAlive
requests to leader.
storage/storagepb: remove watchID from Event
storage: add WatchResponse to watcher.go to wrap events, watchID
storage: watchableStore to use WatchResponse
storage: kv_test with WatchResponse
etcdserver/api/v3rpc: watch to receive storage.WatchResponse type
Watcher vs Watching in storage pkg is confusing. Watcher should be named
as watchStream since it contains a channel as stream to send out events.
Then we can rename watching to watcher, which actually watches on a key
and send watched events through watchStream.
This commits renames watcher to watchStram.
When a watch stream closes, both of the watcher.Chan and closec
will be closed.
If watcher.Chan is closed, we should not send out the empty event.
Sending the empty is wrong and waste a lot of CPU resources.
Instead we should just return.
