[refactor] Change tx_relay structure to be unique_ptr

Summary:
This is a partial backport of Core PR15759 : https://github.com/bitcoin/bitcoin/pull/15759/commits/c4aa2ba82211ea5988ed7fe21e1b08bc3367e6d4

Depends on D6400

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6401