Sabre Update to 2.1

- VObject fixes for Sabre\VObject 3.3
- Remove VObject property workarounds
- Added prefetching for tags in sabre tags plugin
- Moved oc_properties logic to separate PropertyStorage backend (WIP)
- Fixed Sabre connector namespaces
- Improved files plugin to handle props on-demand
- Moved allowed props from server class to files plugin
- Fixed tags caching for files that are known to have no tags
  (less queries)
- Added/fixed unit tests for Sabre FilesPlugin, TagsPlugin
- Replace OC\Connector\Sabre\Request with direct call to
  httpRequest->setUrl()
- Fix exception detection in DAV client when using Sabre\DAV\Client
- Added setETag() on Node instead of using the static FileSystem
- Also preload tags/props when depth is infinity