FBR: Change to category based crawling

[Adrian Reber]

As discussed previously I would like to change the crawler to crawl each
category separately. The goal is to reduce the load on the database by
distributing the crawling better over the whole day and to reduce the
chance of mirrors being disabled because of the high database load.

This should also remove the need for mirror administrators to create
multiple hosts in MirrorManager to work around the 4 hours timeout per
host.

Attached is my patch. Please +1. This affects mm-crawler01 and
mm-crawler02.

		Adrian