Update account UtxoStore after creating transactions.

This removes inputs spent by a transaction from the stored UTXOs.  If
the UtxoStore is modified, all frontends are notified of the new
confirmed and unconfirmed account balances.

More work must be done later to check that the transaction actually
does occur in a later block.  btcd will make a best try effort to
relay the tx to the network, but it is still ultimately btcwallet's
responsibility.  Added a TODO so I remember to do this in the future.