livesync improvements

• livesync: Rename activatemode methods to *_activatemode
  This matches the other plugins.

• livesync: Simplify start_src_task and src_loop
  This should effect no change in behavior.

• livesync: Improve audio duration fixups

  • An entirely missing duration is now only logged at debug level instead of pretending the duration was zero and warning about it.
  • Silently fix up a duration difference up to one sample.
  • Error when we fail to calculate the duration; don't try to apply the fallback_duration to a non-video stream.

• livesync: Log a category error when we are missing the segment

• livesync: Clean up state handling

  • Separate resetting state more cleanly, introducing set_flushing, sink_reset and src_reset.
  • Clear the queue early when we flush, in order to unblock waits on query responses.
  • Return an error when we fail to start, pause or stop the task.

• livesync: Replace an if-let with match
  No change in behavior, yet. Separate commit to ease reviewing.

• livesync: Move a notify closer to the interesting state change
  Move the notify_all to where we pop the buffer. We're moving within a single state lock so no change in behavior.

• livesync: Move num_in counting to the src task
  This is in preparation for moving more accept/discard logic to the src task, so we can only count num_in here.

• livesync: Simplify num_duplicate counting

• livesync: Handle flags and late buffer patching after queueing
  This makes the chain function almost independent of the output state. We still do the early discard check with buffer_is_backwards so we don't try to queue buffers we can't use, allowing us to fast-forward upstream without blocking on the src task.

  Don't accept LateOverThreshold buffers when we have pending_caps or a pending_segment. We need to apply these first before we can sensibly patch buffers from the new stream.

  Deduplicate most of the output buffer patching code into a new patch_output_buffer method.

  For: #450 (closed)

• livesync: Separate out_buffer duplicate status from GAP flag
  Otherwise we might get confused by upstream GAP buffers.

• livesync: Use fallback_duration for audio repeat buffers as well
  Don't depend on upstream giving us sanely-sized buffers if we want to repeat.

• livesync: example: Add identitys single-segment=1
  These let us change the runtime offset of the test buffers via pad offsets without pushing new segments into livesync, which is necessary to demo the late-threshold behavior.