[untracked] implemented service channel b/w consumer and producer

- any metadata can be now distributed using ServiceChannel instance in
SessionInfo structure
- currently, only meta info about ongoing media streams and their
encoding parameters is published
ref #1928 [implemented external capturer in library]
[untracked] hd camera support

- supported external capturers using AVFoundation
[untracked] fixed crash on close

- app was crashing when exiting while publishing
[untracked] proper shutting down in headless mode
[untracked] implemented camera selection in demo app
[untracked] implemented frame software scaling

- now frames are always scaled (bilinear scale) to encode_width X
encode_height size (if these values are present in configuration file)
[untracked] fixed capturing frame rate measuring for external capturers
[untracked] error handling

- added error code checking when starting publishing with external
capturer
[untracked] added testbed hubs list

- added testbed hubs list
- added python script for querying hubs lists for data
ref #1725 [timing assertion failure]
[untracked] small fixes, corrections
ref #1725 [proper flushing jitter timer]
ref #1960 [check for old data after rebuffering and check on RTT estimation]
[untracked] set default KeyChain for consumer audio/video faces
ref1894 [disabled service channel for demo as it's not fully implemented yet]
[untracked] handling socket errors when trying to fetch
[untracked] hotfix

fixed crash while stopping consumer
final demo code
'dev'->'master'