Correlation ID implementation

Handle the correlation ID (hopefully) uniquely generated by mod_bunny to
track checks through their processing. Mostly useful for debugging
purpose.
Honor check message `reply-to` header

If a (non-empty) `reply-to` header is specified in a check AMQP message,
publish the check result using this value as routing key; if not
specified, use the global `publisher_routing_key` setting value.