Implement SequenceLocks functions

SequenceLocks functions are used to evaluate sequence lock times or heights per BIP 68.

The majority of this code is copied from maaku in #6312
Further credit: btcdrak, sipa, NicolasDorier

src/main.h

@@ -341,7 +341,22 @@ bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime);
  */
 bool CheckFinalTx(const CTransaction &tx, int flags = -1);
 
-/** 
+/**
+ * Check if transaction is final per BIP 68 sequence numbers and can be included in a block.
+ * Consensus critical. Takes as input a list of heights at which tx's inputs (in order) confirmed.
+ */
+bool SequenceLocks(const CTransaction &tx, int flags, std::vector<int>* prevHeights, const CBlockIndex& block);
+
+/**
+ * Check if transaction will be BIP 68 final in the next block to be created.
+ *
+ * Calls SequenceLocks() with data from the tip of the current active chain.
+ *
+ * See consensus/consensus.h for flag definitions.
+ */
+bool CheckSequenceLocks(const CTransaction &tx, int flags);
+
+/**
  * Closure representing one script verification
  * Note that this stores references to the spending transaction 
  */