Create new BlockPolicyEstimator and use it as a replacement for the fee estimates from MinerPolicyEstimator

This class groups transactions that have been confirmed into a block 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 by block for each bucket and confirm block count of the percentage of transactions in that bucket that were confirmed withing that number of blocks.
Add tracking of clearance height to CTxMemPoolEntry.

This is the height of the block chain when a transactions inputs are all available and it is clear to be included in a block.
Note this is a small change in behavior from the prior version in that a transaction that was cleared by a given block isn't penalized for not being included in that block and still gets a confirmation count of 1 if it appears in the next block.
Remove CMinerPolicyEstimator and CBlockAverage code