Fixes a few loose ends in the full cycle of sending coins,
accepting them, pushing that transaction to the pool and having it
mined. More specifically:
* The API output endpoint needs to be a UTXO endpoint, as the
server can't make any guarantee about having a spent output.
* Bubbling up HTTP not found errors.
* Wallet output status checker now handles spent outputs.
* Transaction pool validates the transaction before accepting it.
* Fixed the operation API routes.
* Fixed too greedy wallet coin selection loop.
Ignotus Peverell
6523966f9e