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/miner.cpp

@@ -148,6 +148,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         CBlockIndex* pindexPrev = chainActive.Tip();
         const int nHeight = pindexPrev->nHeight + 1;
         pblock->nTime = GetAdjustedTime();
+        const int64_t nMedianTimePast = pindexPrev->GetMedianTimePast();
         CCoinsViewCache view(pcoinsTip);
 
         // Priority order to process transactions
@@ -162,7 +163,12 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
              mi != mempool.mapTx.end(); ++mi)
         {
             const CTransaction& tx = mi->GetTx();
-            if (tx.IsCoinBase() || !IsFinalTx(tx, nHeight, pblock->nTime))
+
+            int64_t nLockTimeCutoff = (STANDARD_LOCKTIME_VERIFY_FLAGS & LOCKTIME_MEDIAN_TIME_PAST)
+                                    ? nMedianTimePast
+                                    : pblock->GetBlockTime();
+
+            if (tx.IsCoinBase() || !IsFinalTx(tx, nHeight, nLockTimeCutoff))
                 continue;
 
             COrphan* porphan = NULL;