MDL-45565 core_message: Fixed strangers array
MDL-45565 core_message: Fixed strangers array

Strangers get mixed with noreply user and support user,
if  noreply/support users real user.
To avoid duplication use userid as array index. Also,
avoid fetching data again for noreply and support users,
if they are real users, then data is already feteched.

dilawar/moodle