SessionState::optimize and SessionState::create_physical_plan Don't Update Query Start Time

[tustvold]

Describe the bug

Calling SessionState::create_physical_plan or SessionState::optimize does not update ExecutionProps::query_execution_start_time and consequently now() will evaluate to the time that the SessionState was created.

To Reproduce

Expected behavior

Having a per-query attribute stored on the SessionState is confusing and error prone, it should be moved to DataFrame

Additional context

These methods will eventually be deprecated and removed as part of #4626

ExecutionProps will also eventually be deprecated and removed as part of #4629