api: add events subscription support

A user can create watcher by the Connection.NewWatcher() call:

    watcher = conn.NewWatcker("key", func(event WatchEvent) {
        // The callback code.
    })

After that, the watcher callback is invoked for the first time. In
this case, the callback is triggered whether or not the key has
already been broadcast. All subsequent invocations are triggered with
box.broadcast() called on the remote host. If a watcher is subscribed
for a key that has not been broadcast yet, the callback is triggered
only once, after the registration of the watcher.

If the key is updated while the watcher callback is running, the
callback will be invoked again with the latest value as soon as it
returns.

Multiple watchers can be created for one key.

If you don’t need the watcher anymore, you can unregister it using
the Unregister method:

    watcher.Unregister()

The api is similar to net.box implementation [1].

It also adds a BroadcastRequest to make it easier to send broadcast
messages.

1. https://www.tarantool.io/en/doc/latest/reference/reference_lua/net_box/#conn-watch

Closes #119

CHANGELOG.md

@@ -11,6 +11,7 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
 ### Added
 
 - Support iproto feature discovery (#120)
+- Event subscription support (#119)
 
 ### Changed