Make RunQueueSpec more stable on slow machines. Fixes #2769

Reduce the minimum amount of concurrency used by at least 10. The actual
level of load should dynamically scale up, so it is OK to start at a
lower baseline.

This change will hopefully make our CI tests more stable.
Merge pull request #2777 from richdougherty/runqueuespec2

Make RunQueueSpec more stable on slow machines. Fixes #2769