Previously the verifier job would scan all transactions in
unverified_tx each time it ran.
Nothing was ever removed from this map; it would essentially
be the full set of transactions.
As the job runs about 10 times a second, for a wallet with 500 txs
this would be 5,000 useless loops a second.
This patch makes unverified_tx be simply the set of confirmed
transactions that haven't yet been verified. txs are added once
confirmed, and removed once verified. Hence it will almost always be
empty.