Cap MTR --parallel per builder and worker

Buildbot's one-size-fits-all rule has been that a builder asking for N jobs (vCPUs) runs MTR with --parallel=2N, and that this is safe on a host where several builds are scheduled. It relies on most of our hosts having about 2 GB of memory per vCPU, and on a test run peaking at about 2 GB per job.

Some builder configurations, such as --big-test or the sanitizers, are outliers and no longer fit this rule.

With this patch:

• Builders keep declaring jobs, which sets the compile parallelism and the job claim on the worker, so they stay fast while compiling.
• A builder can cap MTR --parallel on specific workers:

  get_config(mtr_max_parallel={"<worker>": N})

  • The cap is applied when the build starts, for the worker running it.
  • Workers not listed keep 2 x jobs.
  • A worker name that is not one of the builder's workers fails the config load.