Multiget

This PR adds a new feature to SabreDAV nodes: multiget!

In particular, there are three reports that greatly benfit from this:

* addressbook-multiget
* calendar-multiget
* sync-collection

By pushing 'getting multiple resources' all the way to the backends, things could go a lot faster.

Before when one of those requests would be done, for each request this would end up in a database query. Now those can be grouped in a single query.