Currently, we're doing "sync" in databroker server. If we're going to
support multiple databroker servers instance, this mechanism won't work.
This commit moves the "sync" to storage backend, by adding new Watch
method. The Watch method will return a channel for the caller. Everytime
something happens inside the storage, we notify the caller by sending a
message to this channel.
Storing server version when creating new server. After then, we can
retrieve the version from backend when server restart.
With storage backend which supports persistent, the server version
won't change after restarting.
