All added columns are for future, not yet implemented features

 * crawl_failures: Count the last consecutive crawl failures.
   This can be used to auto disable a host if the crawler fails
   multiple times in a row.
 * disable_reason:  Add a text field to specify why the mirror was disabled.
   This can either be filled by the crawler for auto disable due
   to crawl failures, or by an admin (e.g., ticket number)
 * push_ssh_private_key:  If SSH based push mirroring will ever be implemented
   this field should contain the private key to connect to the destination host
 * push_ssh_host: The host to contact for push mirroring
 * push_ssh_command: The command to execute on the destination host for
   push mirroring
 * last_crawls: This field holds information about the last few crawls.
   Which protocols were used, crawl duration, ...