Reset count on searching

When using the `search` method on a CardDAV backed, consecutive searches reuse the first results count.

This produces some undesired behaviours for some operations. For instance, when importing a .vcf on a CardDAV address book, most of the contacts are skipped if any of them was already present.

This PR just resets the `total_count` attribute when setting a new filter or a new group on the backend.