Don't clear cache so many times

On modr8_form_submit, cache_clear_all is invoked for every approved node.
cache_clear_all is expensive and doesn't need to be called more than once per page view.
This patch creates a flag which, if set, clears the cache once and only once.

This is marked "bug report" because the amount of overhead created by so many cache_clear_all's could cause out of memory errors or http timeouts.