Dispose CancellationTokenSource

This commit disposes the CancellationTokenSource when diposing the BulkAllObservable<T>.
Since the CancellationTokenSource could be linked to a passed CancellationToken
that sets a timeout, not disposing it can lead to flooding the timer queue, affecting performance.

Author: russcam

src/Nest/Document/Multiple/BulkAll/BulkAllObservable.cs

@@ -200,6 +200,7 @@ public void Dispose()
 		{
 			this.IsDisposed = true;
 			this._compositeCancelTokenSource?.Cancel();
+			this._compositeCancelTokenSource?.Dispose();
 		}
 	}
 }