Support storages that do not implement the CS3API Lock interface

[glpatcern]

This is for now food for thought.

In Reva, we are able to support storage drivers on top of WebDAV-based systems, which do not necessarily implement the full CS3API storage provider interface. In particular, the Lock API may be missing or not working.

We may want to detect that (from the response of a SetLock call) and implement an alternative based on xattrs (SetArbitraryMetadata). Surely not race-proof but it might be better than completely bailing out.

[glpatcern]

PoC implementation was done to target ScienceMesh apps for OC and NC, as the Reva storage provider for nextcloud does not implement to date the Lock API (see sciencemesh/nc-sciencemesh#53)