Disqualifies a node when the node fails to complete a graceful
exit.
Adds a new DisqualifyNode method to the overlay cache, since there
wasn't an existing method to disqualify a node but do nothing else
to its stats.
Adds checks to existing tests to make sure that a storage node that
fails a graceful exit is marked as disqualified in the overlay
cache.
https: //storjlabs.atlassian.net/browse/V3-3342
Change-Id: I4d554a519ab59db31ad3b8e28764c8683a6e3888
1cb0f80a8d