Enhance webDAV sync

[tobiasKaminsky]

This is a wrap-up of our call, @rullzer @er-vin @marinofaggiana
If something is missing/wrong please adjust

Problem:

• clients, especially mobile, need to take data/time into account when doing large propfinds, e.g. folder with > 10k files

Webdav Sync collection:

• https://tools.ietf.org/html/rfc6578
• upon each propfind with syncToken: store clientId, syncToken, children with eTag
• upon following propfind with syncToken: compare new state with old and return only diff

Rating:

• reduces transferred data, parsing on client side
• increases load on server
• increases database storage
  • to reduce this:
    • only use this method if > 50(?) items in folder
    • do not use it on Desktop, only mobile clients

Journal:

• store any modification
• compress if no sync token was used