[Reva] Delete expired Public Shares

[refs]

Public Links shares are not yet being deleted when they expire, instead, they're not being returned from the API call. This means you can still access a publicly shared resource as long as you keep the URL. This is a bug.

Useful information:

Transcript from @PVince81:

in OC10 we delete the shares on demand if they are expired. I'd tend to want the logic in shares.go and not json.go

[exalate-issue-sync]

Alex Unger commented: I can take over this one. I opened a PR some time ago about this: cs3org/reva#959

[exalate-issue-sync]

Alex Unger commented: This has been reworked into cs3org/reva#1361

The approach followed is to tackle public share deletion directly on the shares manager instead of the grpc service to spare API calls to delete shares.

Disclaimer: all managers (json + memory) need to be updated, since memory is not used, from this point on, any new public share manager implementation MUST delete shares on list to prevent them to be on the response, and cleanup the "database".