fix: Workaround postconf write settling logic (#2998)

* fix: Workaround `postconf` write settle logic

After updating `main.cf`, to avoid an enforced delay from reading the config by postfix tools, we can ensure the modified time is at least 2 seconds in the past as a workaround. This should be ok with our usage AFAIK.

Shaves off 2+ seconds roughly off each container startup, reduces roughly 2+ minutes off tests.

* chore: Only modify `mtime` if less than 2 seconds ago

- Slight improvement by avoiding unnecessary writes with a conditional check on the util method.
- Can more comfortably call this during `postfix reload` in the change detection cycle now.
- Identified other tests that'd benefit from this, created a helper method to call instead of copy/paste.
- The `setup email restrict` command also did a modification and reload. Added util method here too.

* tests(fix): `mail_smtponly.bats` should wait for Postfix

- `postfix reload` fails if the service is not ready yet.
- `service postfix reload` and `/etc/init.d/postfix reload` presumably wait until it is ready? (as these work regardless)

* chore: Review feedback - Move reload method into utilities

target/scripts/helpers/utils.sh

@@ -44,3 +44,22 @@ function _require_n_parameters_or_print_usage
   [[ ${1:-} == 'help' ]]  && { __usage ; exit 0 ; }
   [[ ${#} -lt ${COUNT} ]] && { __usage ; exit 1 ; }
 }
+
+# NOTE: Postfix commands that read `main.cf` will stall execution,
+# until the config file has not be written to for at least 2 seconds.
+# After we modify the config explicitly, we can safely assume (reasonably)
+# that the write stream has completed, and it is safe to read the config.
+# https://github.com/docker-mailserver/docker-mailserver/issues/2985
+function _adjust_mtime_for_postfix_maincf
+{
+  if [[ $(( $(date '+%s') - $(stat -c '%Y' '/etc/postfix/main.cf') )) -lt 2 ]]
+  then
+    touch -d '2 seconds ago' /etc/postfix/main.cf
+  fi
+}
+
+function _reload_postfix
+{
+  _adjust_mtime_for_postfix_maincf
+  postfix reload
+}