Send keepalive messages if the rtsp server supports this feature

The RTSP servers are programmed to close the session if no proof of client liveness is detected before the timeout expiration.

It seems there is no standard method for this checking. Some servers rely only on the reception of RR packets, some others ignore them and rely on the reception of a keep alive message with the GET_PARAMETER command.

This command is not implemented.I added it to solve an issue when using RTP/Unicast with my AXIS devices.

Note: When using TCP, this may not be necessary to control liveness with this method. This does not cause troubles with my AXIS devices but I notice some inconsistency in documentations (beyond my knowledges).