Pause POS mining while current block is POS

There is no point in creating POS blocks that will
automatically be orphaned by the new trust rules.
Hopefully this will reduce the number of those
annoying POS orphan transactions.

I also slipped in some extra debug output into
ProcessBlock().
POS mining performance. Kernel stake modifiers.

Make POS mining even less burdensome by storing
off the kernel stake modifiers when they are first
calculated and then re-using them.
Merge remote-tracking branch 'origin/pausePOSmining'