check for existing remote observer when adding a new one
[bugfix] fixed critical bug in playout mechanism

- previously, playout mechanism was incorrectly saving frame timestamp
- rather than saving timestamp for successfully played back frame, a
timestamp for any frame (even incomplete) should be saved if given, as
it is used in accumulative inferred playback calculations which may
affect playback adjustments in future - when first complete is received.
made compatible with the latest WebRTC (r7854)

- some methods’ signatures has changed in WebRTC. fixed NDN-RTC to
accommodate these changes.
'dev'->'master'