Make link indexing and sitemap regeneration on cron optional

I'm working on a site with lots of nodes and it is preferable for us to run link indexing and site regeneration on demand through drush instead of on cron. This way we don't need to take the performance hit of indexing xmlsitemap during peak times where it still may be necessary to invoke hook_cron for other tasks.

We want to run indexing and regeneration with drush nightly during off-peak hours through Jenkins/Hudson. Attached is a patch that adds a switch to turn off intensive tasks through cron. The patch should not affect anyone who doesn't change the default value of the switch.