Merge pull request #317 from sortega/issue/113_persistent_exchange

Make exchange transaction publication persistent
Stash is not needed for DefaultExchangeTransactionBroadcaster
Split interface and implementation of the handshake actor
Make handshake actor derive from PersistentActor

But not persist any event yet.
Persist handshake start
Merge pull request #318 from sortega/issue/113_persistent_exchange

Towards exchange handshake persistence
Improve how transaction signatures are logged

As bitcoinjs TransactionSignature has no proper toString, add one in
TransactionSignatureUtils.
Merge pull request #319 from sortega/improve_logging

Improve how transaction signatures are logged