Support watchers (subscription to server events) #309

See Tarantool server events documentation for more info. See go-tarantool patch for reference implementation.

Watchers are expected to cover cases which are covered by box.session.push now. Since the latter is planned to be removed, we need to introduce a proper replacement first.

It is a design question how to introduce asynchronous mechanism to a synchronous connector. One way is to use threads like in connection pool health check.