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/main.h

@@ -308,8 +308,10 @@ bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime);
  * Check if transaction will be final in the next block to be created.
  *
  * Calls IsFinalTx() with current block height and appropriate block time.
+ *
+ * See consensus/consensus.h for flag definitions.
  */
-bool CheckFinalTx(const CTransaction &tx);
+bool CheckFinalTx(const CTransaction &tx, int flags = -1);
 
 /** 
  * Closure representing one script verification