Take the training wheels off anti-fee-sniping

Now that the off-by-one error w/nLockTime txs issue has been fixed by
87550eef (75a4d512 in the 0.11 branch) we can make the anti-fee-sniping
protection create transactions with nLockTime set such that they're only
valid in the next block, rather than an earlier block.

There was also a concern about poor propagation, however testing with
transactions with nLockTime = GetAdjustedTime()+1 as a proxy for
nLockTime propagation, as well as a few transactions sent the moment
blocks were received, has turned up no detectable issues with
propagation. If you have a block at a given height you certainly have at
least one peer with that block who will accept the transaction. That
peer will certainly have other peers who will accept it, and soon
essentially the whole network has the transaction. In particular, if a
node recives a transaction that it rejects due to the tx being
non-final, it will be accepted again later as it winds its way around
the network.