core/account/utxodb: cache confirmed UTXOs
core/account/utxodb: cache confirmed UTXOs

Cache confirmed UTXOs on the sourceReserver. Since they're confirmed,
we're guaranteed that they will stay valid. Also, since the reserver is
only
run on one leader process, no one else can have spent the UTXOs unless
they circumvented reservation.

Closes #201

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain