Signed-off-by: Pesala Silva <pesala.silva@ifs.com> Conference Slot Changes

[PesalaDeSilva]

Preserve audio connectivity between calls even if the media stream is re-established - Extending PJSIP library to automatically remember the states of the audio connections between calls independently of the lifetime of the actual media streams. If a media stream dynamically reinitializes within the progress of a call; then after reconnection, it will automatically restore the connection without the application layer’s involvement.
Likewise, this enables the application to manage the connectivity between calls even when there are no media streams active (like at the very start of the call when early media is disabled on one end). This is primarily implemented with the addition of the following five new methods and updates to the final three:

• pjmedia_conf_replace_port
• replace_conference_slot_to_null_port (internal)
• remove_conf_port (internal)
• allocate_conference_slot (internal)
• deallocate_conference_slot (internal)
• pjsua_aud_stop_stream – replace stream with a null port before terminating
• pjmedia_stream_create – replace existing (null) stream with the new one
• pjsua_call_get_conf_port - modified to always return a usable conf slot if possible