Refresh token of remote server connections

[sevillal]

When connecting to an Azure ML Compute Instance jupyter server the auth token can expire, the Azure ML extension can include an expireOn utc date that can indicate the Python extension when to request a new token.

[greazer]

Let's try using ping to keep alive.

[rchiodo]

Some extra info from Sergio:

Rich, while testing I found that the token does not expire if you keep using the same notebook, but if the token has expired and you open a new notebook then it fails. 

So we should be able to keep it alive with a ping and we need to rerequest when creating a new notebook.