`issueasset` should pick a non-null contract_hash

Grabbing a key from keypool and committing to that public key would allow a simpler method of proving issuance by that person, rather than showing they spent the input that created it.

Currently the only way to do this is the `rawissueasset` API.

Start by locating the issueasset and rawissueasset API entry points and comparing how each obtains or sets contract_hash. Trace the keypool and issuance flow, then verify that issueasset can commit to a keypool public key and returns a non-null contract_hash without requiring the input-spending proof described in the issue.

Written by the indexing model from the issue text.