Add a command to the satellite for cleaning up the Graceful Exit (a.k.a
GE) transfer queue items of nodes that have exited.
The commit adds to the GE satellite DB a couple of new methods, and its
corresponding test, for performing the operations of the new command.
Change-Id: I29a572a59689d63b24990ac13c52e76d65aaa917
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
