Multi-Dispose Of ThresholdLoggingTracer Causes ObjectDisposeException

If a ThresholdLoggingTracer is disposed more than once, it shouldn't error on the second call. This can occur if the Cluster object is disposed more than once.

https://github.com/couchbase/couchbase-net-client/blob/master/src/Couchbase/Core/Diagnostics/Tracing/ThresholdLoggingTracer.cs#L246

Should set _queue to null after disposing to prevent this error.

Note: This issue could also be considered to extend to how the Tracer property is handled on ClientConfiguration. If the same ClientConfiguration object is reused across more than one Cluster, the Tracer is reused though it has been disposed.