Even though we don't continue iterating, this is a cleaner
way of removing the item.

Also, disallow copying of mainbuffer since we haven't implemented
a proper move or copy constructor (and there's no need).

- mixer rendering implemention
=> frame based (was per sources batch based)

- add backward signaling to Observer/Obsevable classes
=> This help mixer to index sources for layout them.

- mutex'ed frame publish (VideoGenerator).

- sinks creation are now done at right places.
=> one per mixer (new), one per camera, one per stream reception.

- VideoRTPSession is fully responsible to handle video pipeline,
between RX/TS streams.
=> exhibit enterConference/exitConference to be aknowledged by upper layers.

- VideoSendThread is not longer a «thread», renamed as VideoSender.

- videoMixer_ is now a shared ptr in Conference objects.
=> getVideoMixer() return a rew shared_ptr also.

- Conference is now responsible to trig video conference pipeline

- std::this_thread::sleep_for() is not usable before GCC 4.1

This should override "receveived" and "rport" parameters, users
who set this are clearly in a situation where they want to force
their network settings.

Use unsigned integers since they are guaranteed to wraparound to 0
when incrementing UINT_MAX.

Note: VideoMixer is not yet ready in this commit.

An audiobuffer of a given duration will return the same number of frames
regardless of how many channels it has.

Fix bug #29818.
Note: non-conference mode has been tested and works.
Conference mode may works, but some issues have been detected.