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
* update lock file and add comment
* add created at and bytes transferred
* cleanup
* rename db func to GetGracefulExitNodesByTimeFrame
* fix flag
* split into two overlay functions
* := to =
* fix test
* add node not found error class
* fix overlay test
* suggested test changes
* review suggestions
* get exit status from overlay.Get()
* check rows.Err
* fix panic when ExitFinishedAt is nil
* fix comments in cmdGracefulExit
