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.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7352

Differential Revision: https://secure.phabricator.com/D11871