Move Optimize onto DataFrame

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The logic to optimize a LogicalPlan currently lives on SessionContext. This creates a couple of problems:

• Forces query_execution_start_time to be stored on SessionState when it is a per-query property
• DataFrame ends up calling back to SessionContext in order to optimize its plans, creating circular coupling between them

Describe the solution you'd like

I originally thought we might need to introduce a new per-query container, but it turns out DataFrame already fits this role nicely, especially following the changes in #4624.

Describe alternatives you've considered

Additional context

#4621 #4622