* change overlay.UpdateStats to allow a third audit outcome. Now it can
handle successful, failed, and unknown audits.
* when "unknown audit reputation"
(unknownAuditAlpha/(unknownAuditAlpha+unknownAuditBeta)) falls below the
DQ threshold, put node into suspension.
* when unknown audit reputation goes above the DQ threshold, remove node
from suspension.
* record unknown audits from audit reporter.
* add basic tests around unknown audits and suspension.
Change-Id: I125f06f3af52e8a29ba48dc19361821a9ff1daa1
