refactor: rename fDoFullFlush to should_write

Summary:
This is a partial backport of core#30611
https://github.com/bitcoin/bitcoin/pull/30611/commits/b557fa7a175f139614932fbb3a4ad0af8271c73c
Depends on D18640

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18641