Add support for multi-pools for workers

We can add different worker pools that are created to match individual job types (e.g. so Plaso can have worker nodes that have more cpu than workers for other job types). Right now it's one global worker pool.

The issue names no files, tests, or entry points. Start by tracing where the single global worker pool is created and how jobs are assigned to workers. Define how job types select separate pools and what configuration and tests are needed to show that pools can use different worker resources.

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