Restore MedianTimePast for locktime.

Revert "Revert "Add rules--presently disabled--for using GetMedianTimePast as endpoint for lock-time calculations""
This reverts commit 40cd32e835092c3158175511da5193193ec54939.

After careful analysis it was determined that the change was, in fact, safe and several people were suffering
momentary confusion about locktime semantics.