Make the new parallel scheduler the default

[acmorrow]

Describe the Feature
Currently, the new parallel scheduler is behind a flag, but with 4.6 released it should become the default in 4.7. Probably the best way to do this is to flip the default for the current experimental flag so there is a fallback in case of trouble, and then hopefully remove the old Parallel in 4.8.

In an ideal world, I'd like to see the new scheduler also be used for builds currently handled by Serial.