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/policy/policy.h

@@ -43,6 +43,9 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
 /** For convenience, standard but not mandatory verify flags. */
 static const unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;
 
+/** Used as the flags parameter to CheckFinalTx() in non-consensus code */
+static const unsigned int STANDARD_LOCKTIME_VERIFY_FLAGS = 0;
+
 bool IsStandard(const CScript& scriptPubKey, txnouttype& whichType);
     /**
      * Check for standard transaction types