Separating max packet size settings for incoming and outgoing in pjmedia

[nanang]

Some UAs may send video RTP packet with size > PJMEDIA_MAX_MTU, this will cause the packet get discarded. A simple solution will be increasing PJMEDIA_MAX_MTU, however this will cause our outgoing RTP packet size will get increased too, which will introduce another issue, e.g: when sending video to other UAs that expects 'normal' MTU. So outgoing and incoming packet size settings should be separated.

Thanks Arkadiusz Wronski for the report.

[nanang]

In 4197:

Fix #1547:

• Added PJMEDIA_MAX_MRU to specify maximum packet size in receiving direction.
• Added max_pkt_size into pj_turn_sock_cfg, default is PJ_TURN_MAX_PKT_LEN.
• Fixed hardcoded MTU settings in media transport SRTP.
• Added MIN_CHUNKS_PER_FRM constant in video stream.

Notes:

• PJMEDIA_MAX_MTU & PJMEDIA_MAX_MRU to specify max packet size for TX and RX dir.
• For ICE media transport, TURN session limits outgoing packet size to PJ_TURN_MAX_PKT_LEN (the size of internal buffer for inserting TURN overheads).