Closing cluster can cause exception in OrphanResponseLogger

Seen this exception in QE situational testing; need to investigate if resources are being cleaned up correctly on cluster close.

2018-09-04 12:43:32,994 [12] ERROR Couchbase.Tracing.OrphanedResponseLogger - Error when procesing Orphaned Responses
System.ArgumentNullException: Value cannot be null.
at System.Threading.SemaphoreSlim.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
at System.Collections.Concurrent.BlockingCollection`1.TryTakeWithNoTimeValidation(T& item, Int32 millisecondsTimeout, CancellationToken cancellationToken, CancellationTokenSource combinedTokenSource)
at System.Collections.Concurrent.BlockingCollection`1.TryTake(T& item, Int32 millisecondsTimeout, CancellationToken cancellationToken)