NCBC-908: Fix connection leak on Memcached bucket dispose

Motivation
----------
Memcached buckets are using HttpStreamingProvider for Couchbase server
node configuration. This implementation did not properly Dispose
underlying ConnectionPools instances.

Modifications
-------------
Dipose is now called on the IConfigInfo implementation after it is released.

Result
------
Memcached buckets no longer leak resources.

Change-Id: Ia88a6f220e76ebda9dc2ae68ff4064eabc57cb6f
Reviewed-on: http://review.couchbase.org/51512
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>

Src/Couchbase/Configuration/Server/Providers/Streaming/HttpStreamingProvider.cs

@@ -304,6 +304,7 @@ public override void UnRegisterObserver(IConfigObserver observer)
                     IConfigInfo configInfo;
                     if (Configs.TryRemove(observer.Name, out configInfo))
                     {
+                        configInfo.Dispose();
                         Log.Info(m => m("Removing config for {0}", observer.Name));
                     }
                 }