At the moment, as soon as someone calls
act_user_manager_get_default() we end up firing of an asynchronous
call to get a list of all users. This is problematic since not all
programs using accountsservice want a list of users.

This commit changes the code to only get a list of users when the
caller invokes act_user_manager_list_users.  This does mean some calls
that were async before are synchronous now, but user proxies were
always obtained synchronously, and they're by far the slowest part
of listing users, so I don't expect this introduce any noticeable
blocking.

Longer term, to fix the sync i/o bits, I think we should
probably ditch libaccountsservice and just make accountsservice use
ObjectManager interfaces over d-bus.