SearchByTreeWalkungMixin ignores sizeLimit

[stohrendorf]

As stated in the title, the mixin ignores the limit, which means that a simple ldap server implementation based on LDAPServer as per the example in the documentation will return too many results if the search filter isn't narrowing enough. In the specific case I'm dealing with, it's pretty easy to get thousands of search results because of the large underlying data base. This puts unnecessary load on the server, the network, and the client.