add a prefix index to filecache.path

The reason that `filecache.path` hasn't had an index added is the mysql limitation of ~1kb for indexeded fields,
which is to small for the `path`, however mysql supports indexing only the first N bytes of a column instead of the entire column,
allowing us to add an index even if the column is to long.

Because the index doesn't cover the entire column it can't be used in all situations where a normal index would be used, but it does cover the `path like 'folder/path/%'` queries that are used in various places.

Sqlite and Postgresql don't support prefix indexes, but they also don't have the 1kb limit and DBAL handles the differences in index creation.

Signed-off-by: Robin Appelman <robin@icewind.nl>