Concurrency & Syncer rework

[maloel]

Following the deadlock that Nir experienced, reworked some stuff (and not all because of the deadlock):

PR #9159:

• split off dispatcher code into dispatcher.cpp
• syncer (timestamp) inactive stream handling moved into skip_missing_stream
• syncer identity_matcher names now include the stream index
• single_consumer_queue::_mutex is now mutable; size() and empty() const
• revised logic inside single_consumer_queue
• revised logic inside dispatcher
• cancellable_time::try_sleep() is now templated, requires actual chrono duration (and not just its ::Rep)
• syncer stop() calls now percolate recursively to shut down all queues; new syncs do nothing if syncer was stopped
• fix 1 warning on the way :)

PR #9210:

• add actual matchers to playback device
• live-test issues with playback
• added !mayfail to frame-number syncing test for stability
• added waiting_on<> in utilities/time/waiting-on.h, with unit-test
• rework dispatcher::flush()

Tracked on [DSO-16888]