Invoke 'Reconnect to kernel' (Websockets) when requesting kernel status through REST calls

[amangarg96]

This enhancement is suggested on facing #516.

Configuration - JupyterLab with Jupyter Enterprise Gateway (in YARN Cluster Mode) to launch remote IPython kernels on YARN-managed clusters.
System - OSX
JupyterLab version - 0.33.12

When the Notebook server loses connection to the kernels, it polls the state of the kernels through REST calls. If it's able to retrieve the kernel state and it's in active/idle state, it does not attempt the reconnect to kernel (websockets), which needs to be invoked explicitly by choosing [Kernels -> reconnect] in classic Jupyter Notebooks and [Reconnect to kernel] in JupyterLab.

(See #516 for the issue this causes)

A reconnect attempt (I think) is a harmless activity and does not interfere with the state of the kernel, so should be invoked automatically.

cc: @kevin-bates