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.
Adds checks to ignore a block that's already known (can happen
if the block was saved but the chain update failed). Also saves
saves the chain slightly differently during bootstrap as the
header chain should only be updated when a header is received
(and not when we got a full block). Finally, do not broadcast
during bootstrap.
Block, Output and Kernel now have bitmasks to hold supported
features and eventually versioning. Will make adding features and
updates easier and open the possibility of soft forks.
First added feature for Output and Kernel is the marking of coinbase
related ones. Allows the validation of the coinbase part of a block.
