prefer to use txindex if available for GetTransaction

Summary:

Motivation: prevent excessive disk reads if txindex is enabled.

This will also benefit to isfinaltransaction.

Backport of core#22383.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12208