Fix leak of iopub object in activity monitoring

After analyzing various leaked items when running either Notebook or
Jupyter Kernel Gateway, one item that recurred across each kernel
startup and shutdown sequence was the a zmq socket instance associated
with the iopub port.  The leaked instance occurs because the activity
monitor and normal port creation logic both call `create_iopub`.

Although this ens up creating a _wrapper_ around the same port, the
object (i.e., wrapper) created by the activity monitor is leaked.  By
setting the kernel manager's `_activity_stream` member to `None` when
the kernel is shutdown, cleanup of the iopub wrapper object will take
place.

notebook/services/kernels/kernelmanager.py

@@ -261,6 +261,7 @@ class MappingKernelManager(MultiKernelManager):
         self._check_kernel_id(kernel_id)
         kernel = self._kernels[kernel_id]
         kernel._activity_stream.close()
+        kernel._activity_stream = None
         self.stop_buffering(kernel_id)
         self._kernel_connections.pop(kernel_id, None)
         self.last_kernel_activity = utcnow()