Don't force consumer queue declaration flags

If no queue name is specified at declaration, the RabbitMQ broker will
generate a unique queue name for the consumer. Therefore, if the queue
name is empty ("") in the mod_bunny configuration, don't declare the
consumer queue with `durable` flag set, and set the `auto-delete` and
`exclusive` flags instead.
Add mb_gen_cid() function
Correlation ID for handled host/service checks

When handling a host or service check, generate a (hopefully) unique
correlation ID by hashing (host name + service description + time), so
that a check can be tracked all the way through its processing by
mod_bunny and a bunny worker. This is mostly useful for debugging
purpose.