Follow-up from "Build circuits preemptively": Make things configurable.

The following discussions from !154 (merged) should be addressed:

• @nickm started a discussion:

  It would be good if this were configurable. (Is it part of the specification?)

• @nickm started a discussion:

  Yeah, these should probably be part of the configuration. But let's put that in after we have the configuration refactored.

• @nickm started a discussion:

  This duration should probably be configurable at some point too.

• @nickm started a discussion:

  Maybe this parallelism should be configurable too, down the road.