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

src/docs/user/configuration/managing_daemons.diviner

@@ -99,8 +99,9 @@ This daemon will daemonize and run normally.
 
 == General Tips ==
 
-  - You can set the number of taskmasters that `phd start` starts by the config
-    key `phd.start-taskmasters`. If you have a task backlog, try increasing it.
+  - You can set the maximum number of taskmasters that will run at once
+    by adjusting `phd.taskmasters`. If you have a task backlog, try increasing
+    it.
   - When you `phd launch` or `phd debug` a daemon, you can type any unique
     substring of its name, so `phd launch pull` will work correctly.
   - `phd stop` and `phd restart` stop **all** of the daemons on the machine, not