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().
Performance boost to POS mining

When there are 250+ inputs eligible
for POS, the yacoin-qt client gets bogged
down by the automatic POS miner. This
performance fix speeds things up by
creating a hash table for the POS miner
thread to use to avoid calculating block
header hashes for every input over and over.
Merge remote-tracking branch 'origin/pausePOSmining'