Option to switch media session to the latest early media SDP received on forked early media

[nanang]

When sending INVITE with media offer and receiving multiple 183 responses (early media) with different To tag, currently PJSIP will only process the first received early media response and discard the other.

There is a report that some system may handover the early media handling after some interval, e.g: from one media gateway to another, by sending another 183 response with different To tag, with different codec and this causes a PJSIP-based endpoint hears silence.

Note that this ticket is not a perfect solution to handle such scenario, for example when the first media gateway keeps sending RTP after the handover, there is still possibility that the stream may switch back to the first media gateway RTP (see also #1366).

[nanang]

In 4441:

Re #1644: Added run-time setting 'pjsip_cfg()->follow_early_media_fork' and compile-time setting 'PJSIP_FOLLOW_EARLY_MEDIA_FORK', the default value is PJ_TRUE.