insights: reduce historical enqueuer delay

Created by: coury-clark

Until we make substantial improvements to our queueing mechanism, there is a quick win to reduce the delay between iterations of the historical_enqueuer to something quite small. This is safe now because:

1. All of the work is now processed in this queue and effectively rate limited
2. Work is blocked while the historical enqueuer is processing, so this will not double up.

I propose 30 seconds between cycles for this job.

This will substantially improve the experience for smaller customers that are making all repo insights.

/cc @joelkw @felixfbecker @vovakulikov @unclejustin