External users with different case username from internal user breaks search for previously-created content

Steps to reproduce

1. Create a user with username "administrator"
2. Login with username "administrator" and create a page A with title 'test1'
3. Configure a LDAP connector
4. Inside the LDAP directory, create a username "Administrator"
5. Make the LDAP directory at the top of the User Directories
6. Create another page B with title 'test2'
7. Confluence Search can only find page B with title 'test2' but cannot find the page A with title 'test1'

There may be other error also.

This error happens because Confluence is using the column user_name in the table cwd_user. Confluence should have used the column lower_user_name. For example, the table content is showing "Administrator" in the column creator for page B.