Reject non-final txs even in testnet/regtest

Previous behavior with IsFinalTx() being an IsStandard() rule was rather confusing and interferred with testing of protocols that depended on nLockTime. A particularly bad example is with CHECKLOCKTIMEVERIFY where it gave the impression that the CLTV opcode didn't do anything as CLTV-using txs that aren't yet minable because the locktimes haven't been reached are accepted.
s/reason/"non-final"