Sometimes nodes who have gracefully exited will still be holding pieces
according to the satellite. This has some unintended side effects
currently, such as nodes getting disqualified after having successfully
exited.
* When the audit reporter attempts to update node stats, do not update
stats (alpha, beta, suspension, disqualification) if the node has
finished graceful exit (audit/reporter_test.go TestGracefullyExitedNotUpdated)
* Treat gracefully exited nodes as "not reputable" so that the repairer
and checker do not count them as healthy (overlay/statdb_test.go
TestKnownUnreliableOrOffline, repair/repair_test.go
TestRepairGracefullyExited)
Change-Id: I1920d60dd35de5b2385a9b06989397628a2f1272