Support timing indicating when cell execution got scheduled

[krassowski]

#29 implements timing of cell execution, but not of when cell execution was scheduled. This can be done by recording the current time when the task to execute the request is put on the task queue, here:

jupyter-server-nbmodel/jupyter_server_nbmodel/handlers.py

Line 569 in 535e241

uid = self._execution_stack.put(kernel_id, snippet, metadata)

or more precisely here:

jupyter-server-nbmodel/jupyter_server_nbmodel/handlers.py

Line 503 in 535e241

self.__tasks[kernel_id].put_nowait((uid, snippet, metadata))

For backward compatibility this should be recorded as iopub.status.busy. This will allow extensions such as jupyterlab-execute-time to display the time since the server-side execution was scheduled (as they do for normal execution).