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.
Merge remote-tracking branch 'petertodd/anti-delorean' into develop

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.