Support Collection Synchronization for WebDAV

[rfc2822]

Support Collection Synchronization for WebDAV (RFC 6578) would allow synchronization of only items which have changed instead of querying the ETags of all items every sync.

Supported by SabreDAV >1.9.0alpha1 and …?

[AutoImport-untitaker]

We also track this with https://github.com/untitaker/vdirsyncer/issues/26, apparently most servers support this now: http://lists.calconnect.org/pipermail/caldeveloper-l/2014-October/000760.html

[rfc2822]

We’re testing Collection Sync support for contacts with the latest DAVdroid development version.

While testing, we have encountered some problems when limiting results (which would by very useful especially for DAVdroid) with certain servers/services:

• https://github.com/nextcloud/server/issues/9339
• fruux (reported over support form on 29 Apr 2018)
• iCloud: works, but ignores limit (should send 507)

(will be updated)

So collection syncs seems to be working with all tested services which support it, but we can’t and won’t use limiting results for now.