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.
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 pull request #34 from grokouser/POSperformance

Performance boost to POS mining