blockstorage: do not flush block to disk if it is already there

Summary:
test: ensure we can reindex from read-only block files now

This is a backport of core#27039

Depends on D18971

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18973