ref #1597 [Implement A/V synchronization v2]

http://redmine.named-data.net/issues/1597
--
- implemented initial approach for a/v synchronization - adapted legacy
code, need testing
'issues/1597-av-sync' -> 'dev'
ref #1616 [Increasing latency depending on hub remoteness]

http://redmine.named-data.net/issues/1616
--
- use the same RTT estimator for both audio and video consumers
ref #1627 [Sometimes video playback stops]

http://redmine.named-data.net/issues/1627
--
- fixed logic for skipping frames: previously, frame could be marked to
be skipped at the acquireSlot call with no respect for the frame type.
this sometimes caused skipping key frames - and NeedKey buffer event
could not be triggered anymore.
fixed by making one frame release point - in releaseSlot call
ref #1616 [Increasing latency depending on hub remoteness]

http://redmine.named-data.net/issues/1616
--
- fixed RTT estimation measurements - now RTT is measured as a time
difference between moments when interest was issued and data has
arrived (including retransmissions). moreover, now RTT is measured for
non-original segments also (i.e. for those that were fetched not from
producer directly) which increases amount of data for estimations
- buffer estimator updated jitter buffer size to be exactly 2*RTT,
where RTT is an estimation. however, buffer estimator prioritize users'
settings and will never make buffer size less than specified in .cfg
file. this feature is under active testing now, may be changed.
- audio/video consumers have their own RTT estimation instances. this
feature is under active testing now and may be changed.
- testing different pipeliner bufferization logic - as it seems that
bufferization mode can be a cause of increased latency. this is under
active testing and will be changed.
- restricted lintiest lifetime to be not less than 250ms
- refactored video playout code
[untracked] fixed crash caused by FEC code

- fixed segmentation fault crash caused by FEC decoding code which
tried to copy memory into null-addressed region
ref #1616 [Increasing latency depending on hub remoteness]

http://redmine.named-data.net/issues/1616
---
- refined and fixed playback timing adjustment code
ref #1616 [Increasing latency depending on hub remoteness]

http://redmine.named-data.net/projects/ndnrtc/issues
--
- adjusted buffering phase
- added more precise playback latency tracking for ntp-synced
producer/consumer
'issues/1616-growing-latency' -> 'issues/1597-av-sync'
ref #1597 [Implement A/V synchronization v2]

http://redmine.named-data.net/issues/1597
--
- now only audio is synchronized around the video in those cases:
 - if audio lags video for more than 45ms
 - if audio leads video for more than 15ms
'issues/1597-av-sync' -> 'issues/1616-growing-latency'
'issues/1616-growing-latency' -> 'dev'
[untracked] fixes for incomplete frames skipping
[untracked] code cleanup
[untracked] code cleanup, refactoring
[untracked] new alpha version v0.8.4