Initialise process pool early

Reduce memory footprint at point of fork to reduce memory footprint at the start of each child process.

Move "process pool size" setting from suite to site/user.
* This allows `cylc run` to not load the suite into memory before the forks.
  As each process starts with the memory footprint of the parent,
  the current logic would created a problem for big suites.
  After this change, memory usage for the workers should become independent of the size of the suite.

See also #1012.