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The session now contains a table of check lists instead of a list and there can now be gaps between check lists.

avoid using functions: av_frame_alloc(), av_frame_unref() and av_frame_free() until libavcodec version 55.45.101

The condition for waking up the V4L2 thread is not good. mblk_t become free when the last filter in the chain no longer uses them.
Furthermore the signal() was done only at fps rate in process method, which was not sufficient.
In order to prevent busy loop in case all buffers are used, an usleep() was added.

Conflicts:
	src/voip/audiostream.c

Conflicts:
	include/mediastreamer2/bitratecontrol.h
	src/voip/bitratecontrol.c
	src/voip/mediastream.c
	src/voip/qosanalyzer.c
	src/voip/videostream.c