LightDM blocks for long time on systems with many users on LDAP server

[Michal Sojka]

LightDM internally maintains a list of all users in the "system". This
is problematic when the user database is on a remote LDAP server and
contains thousands of users, because loading all of them can block
lightdm for minutes. This blocking seems to happen at least three times:
1) when lightdm daemon is started, 2) when a lightdm
child (seat/session?) is started and 3) when a user moves focus in the
greeter (gtk) from username to password field.

This attached patch eliminates long blocking by loading the users only on demand.
The whole test suite passes, but I'm afraid that some functionality
could be broken. Feedback welcome.

[Michal Sojka (sojkam1-fel)]

I'm sorry, I filled this bug against the greeter by mistake. It is probably related to lightdm bug https://bugs.launchpad.net/lightdm/+bug/944041.