Switch to new transaction notifications.

This change removes the processedtx notification, replacing it with
recvtx, and the spenttx notification, replacing it with redeemingtx.
Both new transactions return the full serialized transaction (encoded
in hexadecimal) rather than details about the transaction.

The old txmined notification has also been completely removed as it is
unreliable due to transaction malleability and no code should be
depending on it.