Revert "SpecialContributions: Avoid using 'contributions' slave for getParentLengths"

Actually this won't work, as the LB will see an already open slave and just reuse the same connection.

This reverts commit fdf4b7cbd934f7f8d76de4564c4d4f83d60d8e4d.

Change-Id: I790f5b303fa264ecc36793b8650c79daae367e70