R1:af303f458b9c

Convert taskmasters to use an autoscale pool

Summary: Ref T7352. This is pretty straightforward. I renamed `phd.start-taskmasters` to `phd.taskmasters` for clarity.

Test Plan:
- Ran `phd start`, `phd start --autoscale-reserve 0.25`, `phd restart --autoscale-reserve 0.25`, etc.
- Examined PID file to see options were passed.
- I'm defaulting this off (0 reserve) and making it a flag rather than an option because it's a very advanced feature which is probably not useful outside of instancing.