Add Plugin support for DejaVu Replay bidirectional communication

[vzyrianov]

One method of replaying is to send gaze data only when all the connected plugins are ready for it by communicating bidirectionally.

In the bidirectional mode, Dejavu sends gaze data, then waits for a response from all the plugins. Once it receives that response, it continues with the replay.

/scripts/test_socket_client.py shows how a plugin should behave (once the sock.sendall(b'a') line is uncommented).