lib-storage: Avoid fsyncing subscriptions file when it doesn't change or if mail_fsync=never.
lib-storage: Avoid fsyncing subscriptions file when it doesn't change or if mail_fsync=never.

jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch