5 different recorders possible. 'last' missing thing on liquidsoap side: smooth transition between mixer.selects

recording is now part of engine.liq. a redefinition of the recorded source is needed if those two should be separated.

This reverts commit 62a8c7ba, reversing
changes made to a9c94736.

several things: critical socket threading bug from comba resolved. adapted interfaces to pv / tank / database

started to implement trackservice. removed unused (comba)methods from scheduler.py&calender.py. minor beautyness improvements