Improve global search performance

Indexing contents is an expensive task and even dangerous in big moodle sites, we should refine the performance as much as we can. This can be considered a blocker.

Possible actions:

• Tune permission checkings
• Use get_fast_modinfo()
• Indexing database fields
• Specific configuration to run search/cron.php (http://docs.moodle.org/dev/Task_API) to avoid blocking other cron tasks.
• MUC