[refactor] Move tx relay state to separate structure

Summary:
This is a partial backport of Core PR15759 : https://github.com/bitcoin/bitcoin/pull/15759/commits/4de0dbac9b286c42a9b10132b7c2d76712f1a319

Depends on D6399 and D6398

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Subscribers: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6400