Even though most of the time these modules will be used by
connectors, there are exceptions (namely, `emqx_rule_engine`).
Besides, they are general enough to land there, more so given
that `emqx_placeholder` is already there.
Due to race conditions, it's possible that the caller to `pmap`/`nolink_apply` might
receive a late reply.
e.g. when a timeout occurred while resource manager was checking a resource's health:
```
19:18:23.084 [error] [data: ..., event_data: {#Reference<0.3247872820.3887857670.131018>, {:normal, [false, true, true, true, true, true]}}, event_type: :info, msg: :ignore_all_other_events, state: :connected]
```
Using an alias and also checking for the race condition in the `after` block (like
[`gen`](a76bf63197/lib/stdlib/src/gen.erl (L270-L277))
does), we avoid polluting the caller's mailbox with late replies.
Which periodically inpects managed certificates directory and tries
to collect "orphans" here, in other words files that aren't
referenced anywhere in the current emqx config.
Andrew Mayorov