audio/video synchronization

- implemented intuitive approach for audio/video synchronization
mechanism
'issues/1109-av-sync' -> 'dev'
ref #1145 [High CPU load]

http://redmine.named-data.net/issues/1145
--
- dedicated thread for responding to FreeSlot events in frame buffer
- additional lock event for playout thread
'issues/1145-high-cpu-load' -> 'dev'
ref #1107 [Upon start fetching or publishing rendering window goes black]

http://redmine.named-data.net/issues/1107
'issues/1107-black-render' -> 'dev'
[untracked bugs]

- fixed issue with synchronization video -> audio
- fixed wrong frame buffer event handling