SignTransaction will be called multiple times in the future. Pass
it a result UniValue so that it can accumulate the results of multiple
SignTransaction passes.
rpc/rawtransaction.cpp moves to libbitcoin_server since it should not be
accessed by non-node libraries. The utility following utility methods
move to their own unit rpc/rawtransaction_util since they need to be
accessed by non-node libraries:
- `ConstructTransaction`
- `TxInErrorToJSON`
- `SignTransaction`