Various updates and fixes for Latency Compensation

Various updates and fixes for Latency Compensation

* centralize signal_latency_at_***_position to processors
* update initial-delay/roll-delay when processor order changes
* consolidate signal-latency calculation: use the same method
  for processor-changes and session's post_playback_latency.
* include relative output-delay in roll-delay
* fix capture processor position & optimize stem-export latency

(roll-delay fixes pending Route:roll() update)