Lock ordering issue in video conference bridge

[nanangizz]

Sample scenario

• Video conference (VC) worker thread: put/get_frame() publishes media event (synchronously), currently put/get_frame() is called while holding the VC mutex and app receiving the event invokes a SIP dialog API which tries to acquire the SIP dialog mutex.
• Other thread, app receives a SIP dialog callback (note: such callback is usually invoked while holding dialog mutex), it invokes a VC API which tries to acquire VC mutex.

The scenario involves non-uniform lock ordering between VC and SIP dialog mutexes which can cause deadlock.

Proposed solution

The proposed solution is to avoid holding VC mutex while calling put/get_frame() in VC worker thread. To allow this, a couple major modifications are implemented in this PR:

1. VC internal states (such as port connection, rendering states) must not be changed when put/get_frame() is called, so some VC API operations (such as connect/disconnect ports, remove port, update port) must be queued or done asynchronously, the real operation will be performed in the VC worker thread context.
2. As remove port operation will be done asynchronously, media ports (especially video ports for now) need to have a reference counter (or group lock) to avoid premature destroy.

Other fixes

This PR also contains a few minor other fixes such as:

• in pj_grp_lock_create_w_handler(): memory pool used for adding handler, updated to using group lock's memory pool itself, previously using app pool, was getting a premature destroy issue because of this (i.e: VC port added using PJSUA vidwin pool, the vidwin may get destroyed before the VC port, group lock crashed when iterating handlers).
• in PJMEDIA stream, somehow transport is accessed before the attach happens and it is still NULL, or it is accessed after set to NULL (in destructor), so put NULL transport check in some RTP/RTCP sending.
• in VC, fill buffer with black is performed when buffer is initially allocated and sink buffer after sink format changed, was every time render state is updated.
• add/modify logs in a few places including in app.

[sauwming]

Also, can you merge with the latest master so I can do some local testing?

[nanangizz]

Also, can you merge with the latest master so I can do some local testing?

Done, hopefully I didn't do it wrong, the traces above look a bit strange :)