After renaming a user, the Avatar service fetches by user name instead of user key

Steps to reproduce:

1. Create a user with a certain username (such as dave)
2. Change the user's username to dave!
3. Add a custom avatar to this user's profile
4. Create a new user with the username dave
5. After a hard refresh, the avatar for dave! is broken

The reason is because the ViewUserAvatarServlet fetches users by username instead of key:

public class ViewUserAvatarServlet extends AbstractAvatarServlet {
    @Override
    protected void defaultDoGet(HttpServletRequest request, HttpServletResponse response, String ownerId, Long avatarId, Avatar.Size size)
            throws IOException, ServletException {
        final AvatarService avatarService = ComponentAccessor.getAvatarService();
        final ApplicationUser avatarUser = getUserUtil().getUserByName(ownerId);
        final ApplicationUser remoteUser = getAuthenticationContext().getLoggedInUser();

        if (avatarUser != null && remoteUser != null && avatarService.isUsingExternalAvatar(remoteUser, avatarUser)) {
            redirectToExternalAvatar(response, ownerId, size);
            return;
        }

        super.defaultDoGet(request, response, ownerId, avatarId, size);
    }

Expected Results

The avatar should display correctly.

Actual Results

The avatar is displayed as a broken image. Attempting to navigate to the avatar directly produces a circular redirect error.

Symptoms:

1. The original username appears in the avatar table, and points to the correct avatar. Permissions on the disk are correct for the avatar.
2. The original username appears twice in the app_user table - one row contains the value in user_key and the other contains the value in lower_user_name

Example:

id    user_key lower_user_name 
----- -------- --------------- 
10105 dave     dave!           
10106 ID10106  dave            

Workaround

You can attempt to remove the value for the lower_user_name that shares the original username:

1. Shut down JIRA
2. Perform a database backup
3. Delete the row from app_user
4. Start JIRA again