btcd now has only one notifier mechanism for received transactions to
a watched address. This previously combines the seperate notifiers
for generic tx and exclusively unspent outputs.
A new handler will be added to watch for spent outputs instead of
relying on the now-removed "btcd:sendtx" notification.
Each wallet needs its own handler running and listening to replies
from btcd, so that wallet can be synced to disk with the new tx or
utxo information. Another rounter to map from addresses to wallets
could have been written, but we'll use the JSON Id router message
router instead.
We must instead wait for the user to explicitly generate their own
wallet for an account name. This is because all btcwallet wallets
must be encrypted, and the passphrase must be known at time of wallet
generation.
