daemon: Wait for reload before servicing list_cached_users

When /etc/passwd, /etc/shadow or /etc/group are changed outside of
AccountsService, the cache reload is delayed by 500 ms so subsequent
changes to these files are process seen together and AccountsService has
a consistent view of the data (since after one of these files is changed
the others may change too).

If ListCachedUsers is called in this 500 ms window,
finish_list_cached_users will be executed before reload_users_timeout
has been dispatched, since its added to the mainloop as an idler and at
point there is nothing preventing it from being executed. This makes
finish_list_cached_users only be attached to the mainloop after
reload_users_timeout has been dispatched.

This bug was introduced by commit 4e3fad33 when the 500 ms delay was
implemented.

Closes: #71
1 job for master in 2 minutes and 30 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #245384
build-fedora

00:02:30