CSync: Simplify csync_owncloud stat/resource handling
CSync: Remove owncloud_stat

We get already all metadata with the readdir
CSync: Use context instead of global variable
CSync: Remove one VIO layering
CSync: We can access the context directly
CSync: Remove old defines
CSync & Propagator: Support a direct download URL

This is for server file backends that support sending a
direct URL.
CSync: Rename callback functions
CSync: Shorten WebDAV property name
Merge pull request #1839 from owncloud/directDownload

Direct download URL support + global variable refactoring

Reviewed offline by @dragotin