Support providing watch_id in WatchCreateRequest

[berteundamman]

Recent versions of etcdv3 allow specifying watch_id in WatchCreateRequest, as described here. This would allow attaching multiple watches in parallel. It would be nice if you could tell the client to use this functionality and not always do it sequentially through AttachQueue.

[berteundamman]

While trying to implement this in etcdv3 I found that etcd grpc proxy doesn't properly support the watch_id yet, so when using a proxy you need to synchronously subscribe anyway. This makes this feature probably a lot less useful until etcd proxy is fixed.