Implement 'pause' command

Implement 'pause' command

The CECP 'pause' an 'resume' commands are implemente, and announced in
the corresponding feature. Commands in paused mode are ignored, except
'quit'. The ReadClock() function has been adapted to allow initialization
of the startTime with an offset, to give the paused time back to the clock.
For this the nodeCount (which naturally stops incrementing during pauses)
has to be reset outside ReadClock again.