Support pre-warming kernels

[DonJayamanne]

This was turned off, and issues that exist:

• When auto start is on and we run a cell immediately ther'es no progress indicator for starting kernels
  • Because auto start is still busy and that is silent
• Select kernel does not put up the 'install ipykernel' message #5736
• Pre-warm daemons (if not done)
• Pre-warm preferred & active environments
• Notes
  • Pre-warm only for local
  • We need to refactor before we start this (thats why this was originally disabled and deferred)

[DonJayamanne]

Testing

• We pre-warm kernels, now, running cells should be slighly faster (as the kernlel is started when the notebook is opened).
• Again, you might want to compare against stable to see an improvement.