Improve reference counter

As part of a investigation in production, we suspected there was an issue with the reference counter of a specific project.

For more context please read: https://gitlab.com/gitlab-com/gl-infra/production/issues/935#note_276022263

The problem is that we don't expose any method to reset the reference counter. Also there is no way to see how long the TTL for releasing the counter should be (we could use that information in a few places to, for example, try an operation again when that TTL reaches zero).