Handle metadata operations for shared folder

[labkode]

Fixes #495

Operations:

• stat
• list container
• move
• delete
• mkdir
• upload
• download

Fixes also #611

ETAG generation for shared folder covered in:
#612

Allows to:

[gonzalhu@iota reva]$ reva ls /home/MyShares/
[gonzalhu@iota reva]$ reva share-update-received -state accepted 2a023c6b-1bd1-4284-a7d2-343c7ca092af
OK
[gonzalhu@iota reva]$ reva ls /home/MyShares/
photos
[gonzalhu@iota reva]$ reva ls /home/MyShares/photos/
Vigo
test

[gonzalhu@iota reva]$ reva mkdir /home/MyShares/photos/Ourense
[gonzalhu@iota reva]$ reva ls /home/MyShares/photos/
Ourense
Vigo

[labkode]

@butonic can you review the current state, I prefer to merge and create another PR for the upload and download parts like that we can already mount/unmount access shares in the user home.

[butonic]

@refs this is related to the /s/ prefix you were using for the public shares provider. Adding a GetPublicShares to the cs3 api seems to leak implementation details AFAICT ...

This PR hardcodes the /home path in the global namespace.

[butonic]

Good for now. We have a different vision that uses spaces / treats shares as first class storages ... but we can discuss and pursue that regardless of this implementation.

[labkode]

@butonic merged, I also add the upload/download methods.