Reduce Priority Cache Grace Confirmations

Could set this even lower as it is only relevant to the priority caching, so even if something goes wrong because of a huge reorg.
Increase MIN_RELAY_TX_FEE ("Bandaid")

MIN_RELAY_TX_FEE = 10000
to
MIN_RELAY_TX_FEE = MIN_TX_FEE/5  --->  100000
Merge pull request #162 from domob1812/simpler-priority

Add "mild" caching for priority calculation.