Add reference transaction hash to Merkle tree for atomic swaps

[mda]

Now every transaction is represented by one hash in Merkle tree. Also current atomic swap implementation is bulky and slow.

With two hashes for transactions involved in atomic swaps the first hash is the usual bitcoin transaction hash and the second one is the hash of the correspondent transaction on an altcoin. In order to spend from this kind of transactions the altcoin transaction has to be already included in its blockchain with n confirmations. So this means that bitcoin miners will monitor the altcoin Merkle tree as well.