metabaseSegmentKey` TransferQueueItem
We are unifying which name (and type) we are using for value we are
using to point to segment. We want to use `key` instead of `path`.
Dedicated type `metabase.SegmentKey` was created for this purposes also.
This change is doing refactoring around gracefulexit.
Change-Id: I90d51ff087b206179e61d5f1bc95f4709d76f917