Create new BlockPolicyEstimator for fee estimates

This class groups transactions that have been confirmed in blocks into buckets, based on either their fee or their priority.  Then for each bucket, the class calculates what percentage of the transactions were confirmed within various numbers of blocks.  It does this by keeping an exponentially decaying moving history for each bucket and confirm block count of the percentage of transactions in that bucket that were confirmed within that number of blocks.

Also eliminate txs which weren't clear at entry from fee/pri calcs
Remove CMinerPolicyEstimator and CBlockAverage code
New smartfees.py and associated funcs in util.py