Add rules--presently disabled--for using GetMedianTimePast as endpoint for lock-time calculations

The lock-time code currently uses CBlock::nTime as the cutoff point for time based locked transactions. This has the unfortunate outcome of creating a perverse incentive for miners to lie about the time of a block in order to collect more fees by including transactions that by wall clock determination have not yet matured. By using CBlockIndex::GetMedianTimePast from the prior block instead, the self-interested miner no longer gains from generating blocks with fraudulent timestamps. Users can compensate for this change by simply adding an hour (3600 seconds) to their time-based lock times.

If enforced, this would be a soft-fork change. This commit only adds the functionality on an unexecuted code path, without changing the behaviour of Bitcoin Core.

src/consensus/consensus.h

@@ -13,4 +13,13 @@ 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),
+};
+
+/** Used as the flags parameter to CheckFinalTx() in non-consensus code */
+static const unsigned int STANDARD_LOCKTIME_VERIFY_FLAGS = 0;
+
 #endif // BITCOIN_CONSENSUS_CONSENSUS_H