NCBC-601: AOoR in Couchbase.Core.VBucket.LocatePrimary() after failover nodes

Change-Id: Ieb64dbcf066cb6b1fca28642399025f69ab81632
Reviewed-on: http://review.couchbase.org/40871
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>