Enable policy enforcing GetMedianTimePast as the end point of lock-time constraints

Transactions are not allowed in the memory pool or selected for inclusion in a block until their lock times exceed chainActive.Tip()->GetMedianTimePast(). However blocks including transactions which are only mature under the old rules are still accepted; this is *not* the soft-fork required to actually rely on the new constraint in production.

src/miner.cpp

@@ -160,7 +160,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
 
             // Enforce sequnce numbers as relative lock-time
             // only for tx.nVersion >= 2.
-            int nLockTimeFlags = 0;
+            int nLockTimeFlags = LOCKTIME_MEDIAN_TIME_PAST;
             if (tx.nVersion >= 2)
                 nLockTimeFlags |= LOCKTIME_VERIFY_SEQUENCE;