Multiple DbContexts Get Dispose Errors

[brantburnett]

If multiple DbContexts are created which share a set of options, calling Dispose on the second context gets an exception. This is because they share the same ClientConfiguration object, which means they share the same ThresholdLoggingTracer, which gets disposed by the first DbContext. Relates to https://issues.couchbase.com/browse/NCBC-2019

Exception message: System.ObjectDisposedException
  The collection has been disposed.
  Object name: 'BlockingCollection'.
Stack trace:
   at System.Collections.Concurrent.BlockingCollection`1.CheckDisposed()
   at System.Collections.Concurrent.BlockingCollection`1.CompleteAdding()
   at Couchbase.Tracing.ThresholdLoggingTracer.Dispose()
   at Couchbase.Cluster.Dispose(Boolean disposing)
   at Couchbase.Cluster.Dispose()
   at Microsoft.EntityFrameworkCore.Couchbase.Storage.Internal.CouchbaseClientWrapper.Dispose()
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.Dispose()
   at Microsoft.EntityFrameworkCore.DbContext.Dispose()

Steps to reproduce

Run query tests in EfCore.Couchbase.FunctionalTests

Note: Getting to this error requires some additional code changes to successfully create buckets when query tests are run, etc. Commits are forthcoming to put these in place, but I wanted to track this bug before I forgot.