eosfs: allow for a way to disable versioning upon saving a file

[jessegeens]

Some applications have autosave features that should be treated differently than a normal save (initiated by the user). One example of this is Jupyter, which only expects a new version if the user manually clicks on the save button. We could apply a similar logic to the office apps as well.

We should extend the save mechanism in Reva to allow us to disable versioning in such cases. 

This could mean a change in the CS3 API's to make it a parameter (as reported in cs3apis#97), but for now, we agreed to use an opaque field for simplicity.