rpcserver: Optimize JSON raw tx input list create.
rpcserver: Optimize JSON raw tx input list create.

This optimizes the createVinList function which is used to generate the
JSON list of transaction inputs.  It also makes it more consistent with
the createVinListPrevOut function.

In particular, it entails the following changes:
- Only do a single coinbase check and return right away instead of
  checking multiple times inside the loop over the inputs
- Use a pointer for populating the details of each entry to avoid
  multiple unnecessary array lookups and bounds checks
- Group all fields that populate the entry for better readability

decred/dcrd,decred/dcrd,jrick/dcrd,jrick/btcd,jrick/btcd,jcmincke/dcrd,jcmincke/dcrd,jrick/btcd,jcmincke/dcrd,jrick/dcrd