[Chronik] Refactor QueryTxs::tx_by_id: remove Cow etc.

Summary:
By inlining make_tx_proto in the match arms we don't need Cows anymore, and the code becomes simpler and more readable.

Note: No actual cows have been harmed in the production of this diff.

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13768