Modify the stream to maintain RTP sequence number and timestamp from the last transmission when it's restarted.

[nanang]

See ticket #600 (Solution 1).

[nanang]

In r2241:

• Introduced new API pjmedia_rtp_session_init2() to enable intializing RTP session with non-default initial settings
• Updated stream so it can be created with non-default initial RTP settings.
• Updated pjsua-lib to make sure RTP timestamp and sequence contigue when stream session is restarted.

[bennylp]

In r2244:

• fixed minor warning about conversion from int to unsigned char

[nanang]

In r2245: fixed assertion of invalid stream index supplied to pjmedia_session_get_stream_stat().

[bennylp]

Done, closing the ticket.