Restore MedianTimePast for locktime.

Revert "Revert "Add rules--presently disabled--for using GetMedianTimePast as endpoint for lock-time calculations""
This reverts commit 40cd32e835.

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

src/consensus/consensus.h

@@ -13,4 +13,10 @@ static const unsigned int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;
 /** Coinbase transaction outputs can only be spent after this number of new blocks (network rule) */
 static const int COINBASE_MATURITY = 100;
 
+/** Flags for LockTime() */
+enum {
+    /* Use GetMedianTimePast() instead of nTime for end point timestamp. */
+    LOCKTIME_MEDIAN_TIME_PAST = (1 << 1),
+};
+
 #endif // BITCOIN_CONSENSUS_CONSENSUS_H