Sync up use of asset labels and RPC calls

Now that asset labels are no longer stored in the wallet, they can be integrated into raw transaction rpc apis.

Currently they are not. This means if a call such as listunspent is changed to return an asset label when set, `createrawtransaction` can't deal with that switch gracefully.

Start by tracing the listunspent and createrawtransaction RPC paths and how asset labels are represented after being removed from wallet storage. The work is done when a label returned by listunspent is accepted gracefully by createrawtransaction and the relevant RPC behavior is covered by tests.

Written by the indexing model from the issue text.