Remove pause feature.

- Wasn't being used very much.
- The background workers should only be using idle cpu, so shouldn't
  have any outward effect.
- Could also use less workers if necessary.

tests/worker_test.py

@@ -30,7 +30,7 @@ class WorkerTestCase(unittest.TestCase):
     def test_run_job(self):
         loop = asyncio.get_event_loop()
         compression = "none"
-        worker_ = worker.Worker(False, False, compression)
+        worker_ = worker.Worker(False, compression)
         loop.run_until_complete(worker_.create_process())
         worker_.process.stdin.write(f"{compression}\n".encode("utf-8"))
         future = worker_.run_tool("foo", tools.metadata)