#36 proof process thread priority

[Leo Freitas]

We've added a two-level priority thread config: higher for ZEves proof; lower for Analysis of proof process.
The ZEves proof thread should always override the proof process analysis thread (preemptively).

This is okay when proving the theorems themselves once finished (e.g. proof rerun), but not during proof development.
That's because often we go back and forth within the same proof, after some time "thinking". At that time, the Analysis
of proof process thread kicks in and if itdelays, the ZEves proof thread doesn't come to the top :-(
Does it make sense?

L