prototype new latency compensation schema

Align audio on routes (tracks & buses) according to
internal up/downstream latency.

TODO:
 * align to global offset & recording position
 * aux-send latency compensation
 * MIDI delay & latcomp
 * remove/supersede old API functions
 * link to jack-port latency
 * documentation
 * ...
latcomp: x-fade delayline on flush