Automatically populate updated values in $USER when user_update_user is called

I've revised this issue a little from the original description:

Steps to reproduce:

1. Login as an admin and student in two different browser windows
2. As admin edit the student's profile - change the firstname, to add a '1' to the end of it
3. As the student go to profile

Expected results:

1. The users firstname should be the same in both the user menu and the profile pages

Actual results:

1. The user menu ($USER) displays out of date information

This is a fairly niche issue, however it might have more serious consequences if $USER gets too far out of date. Note that we have manual workarounds for this in user/edit.php for the currently logged in user.

Original description:

As seen in MDL-50162, when a user edits their own profile/properties we need to manually update the $USER object which is stored in the session. Failing to do so appears as if the value wasn't changed.

This can also potentially be a security concern as an admin that would update a field from another use who's currently logged in will not see the change happening.

I suggest we either:

1. Invalid the cache in the session
2. Implement auto update in user_update_user