Optimize redirect_fetch_rids_by_path

The query in redirect_fetch_rids_by_path() cannot use and index to match the source, but this is extremely slow for sites with a large "redirect" table. Even with an index, a case-insensitive LIKE would be slow on non-MySQL databases.

Here is my suggested fix (which is portable to non-MySQL and still fast):

1. Add an index that starts with "source." It can include other columns, but it needs to start with "source."
2. Update redirect_path_update() -- and anywhere else that writes to the "source" column -- to coerce the source to lowercase.
3. Create a schema update that coerces existing "source" column data to lowercase.
4. Update redirect_fetch_rids_by_path() to coerce $source to lowercase before using it in the query. Drop the LIKE.

You could also coerce to uppercase. Either would work.

A quick fix, which would at least make things on MySQL with case-insensitive indexes (the Drupal+MySQL default) fast, would be to create an index on (source, language).