This supplies a push notification on txid becoming known to the Hub.
Additionally this changes its close relative the AddressMonitorService
to use a different tag for the transaction data and the
double-spend-proof data so as to make it obvious which one is being
sent.
In the address monitor service this detects specifically if a double
spend detected by the Hub is applicable for our subscriber, and if it is
then a Message will be sent to the subscriber.
This introduces a new message type DoubleSpendFound
and changes the API for the TransactionFound message while dropping the
TransactionRejected message.