add a Merkle commitment to transactions in block header (algorand#1879)

This will enable short proofs of transactions being present in a given
block.

The proof includes both the transaction ID (which is likely the most
useful part, knowing that some transaction ID was committed), as well as
the hash of the entire SignedTxn and ApplyData (in case some application
wants to know what LogicSig.Args were used, or what the ApplyData was
for rewards calculation).

The Merkle tree uses the same merklearray code that's already used for
compact certificates.

config/consensus.go

@@ -887,6 +887,9 @@ func initConsensusProtocols() {
 	vFuture.CompactCertWeightThreshold = (1 << 32) * 30 / 100
 	vFuture.CompactCertSecKQ = 128
 
+	// Enable transaction Merkle tree.
+	vFuture.PaysetCommit = PaysetCommitMerkle
+
 	Consensus[protocol.ConsensusFuture] = vFuture
 }