crimson/os/seastore: introduce stage_visibility_handoff

crimson/os/seastore: introduce stage_visibility_handoff

Generalize prepare_rewrite_publish_to_prior() into stage_visibility_handoff().

* introduce should_use_no_conflict_publish

* Replace is_rewrite_transaction() checks with should_use_no_conflict_publish(),
  so adding new no-conflict users becomes straightforward.

* Stop committing metadata (commit_state + sync_checksum) during prepare_record()
  (pre-commit). While it is correct for rewrite, doing it pre-commit doesn't buy
  us anything today because readers are still blocked until the publish finishes.

  Moving metadata commit to the after commit phase would also make future
  non-rewrite users easier to support.

This is a prep step for expanding no-conflict publish coverage.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>