coins: GetTxFees method
txmempool: lookupConflicts method
Introduce CNodePolicy for putting isolated node policy code and parameters on

Initially populated with policy-specific code moved from AcceptToMemoryPool.
SQUASHME: actually use GetTxFees method
SQUASHME: Get rid of CNodePolicy::fRequireStandardTx
Use a factory with policy as we do with chainparams
Separate Standard and testing policies [Get rid of Params().RequireStandard()]