Fix kolab cache sync issues

Summary: Use QRESYNC, get rid of "scheduled" cache reset, other small improvements

Reviewers: #roundcube_kolab_plugins_developers, vanmeeuwen

Reviewed By: #roundcube_kolab_plugins_developers, vanmeeuwen

Subscribers: vanmeeuwen, #roundcube_kolab_plugins_developers

Differential Revision: https://git.kolab.org/D1726