Lower the basic job runner parallelism

This patch has two primary aims:

- Reduce the number of concurrent database accesses in a 'thundering herd'
  scenario when a large refreshLinks job comes in. See ops list thread "enwiki
  outage 2014-09-19 08:50:00".

- Improve efficiency by avoiding excessive context switching. Aim to roughly
  match the runnable processes to the available hyperthreads on these
  machines, which is currently 24 threads. For this we count the parsoid jobs
  as the equivalent of 1-2 cpu bound jobs, as all they do is do http requests
  to parsoid and wait for the result. In practice the number of runnable jobs
  including non-php processes was hovering around 35 on these machines, so
  this reduction should result in a less aggressive oversubscription. Also see
  ops list thread "Job runner CPU usage almost tripled over last months".

Change-Id: I9c3a39ba4429f90767ef02a8d8a5e064c8928175