[tests] fix: CTerracoinSecret data type.
[core] transaction,notifier: posted data now signed.

 - first http notification for a given transaction now include a new
   json document, providing message signature for the first json
   document.
 - json transaction data is signed using the existing signmessage()
   function (ECDSA), so any remote client may verify signature, without
   the need for extra keys/data.
 - https://github.com/terracoin/terracoin/wiki/TransactionNotifier
   should be updated soon.