Save/restore mempool
New relay policy
Refactor AllowFree(dPriority) --> CTransaction::dMinFreePriority

Also moved CTransaction static vars from main.cpp to core.cpp
And added new knob for miners: -minfreepriority to override
default COIN*144/250.
remove GetMinFee, replace with mempool.estimate
Un-harcode the "is dust" test, use mempool estimate of min fee
Remove CTransaction::nMinRelayTxFee