rtp: Fix sending of small packets
The current implementation for RTP send isn't optimised for sending MTU bytes of data like rtp-native. For eg. if MTU is 1280 bytes and we have to send 1276 bytes, two packets are send out one of 1268 bytes and other of 8 bytes. Sending out a packet of 8 bytes has a significant overhead and we should be sending MTU bytes of data.
Fix this by accumulating MTU bytes of data and sending data only on accumulation of MTU worth of data.