Add cron warm-up feature

Problem/Motivation

Rather than waiting for the first hit after a node change, we could pre-generate the sitemap via hook_cron() to improve crawl performance and avoid a delayed responses.

Steps to reproduce

N/A

Proposed resolution

Add cron warm-up feature

Remaining tasks

Add cron warm-up feature

User interface changes

N/A

API changes

N/A

Data model changes

N/A