rtspsrc: Use a mutex for protecting against concurrent send/receives
We currently send data to the RTSP connection from multiple threads: whenever a command is to be handled and whenever RTCP is generated. This can cause data corruption or worse if both happen at the same time. As such, protect gst_rtsp_connection_send() and gst_rtsp_connection_receive() calls with a mutex. While this means that we hold a mutex during the IO operation, this is not actually a problem as the IO operation can be interrupted (gst_rtsp_connection_flush()) at any time and is blocking by itself anyway.