Create presets for common needs: max throughput, minimum latency, etc

[benjie]

Feature description

• Max throughput would prioritise throughput over latency: enable batching, use refetch delay (see: Reduce DB load on near-empty jobs table with frequent add_job #500), etc
• Min latency would prioritise latency from filing a job to executing it: minimal batching, no refetch delay, etc
• High scale might be a balance of these suitable for people who are dealing with very high numbers of workers/jobs.

Motivating example

People don't like to read documentation or have too many options; pick some sensible ones for them.

Breaking changes

None.