Improved ratelimit ACL condition.

Improved ratelimit ACL condition.

Replace /noupdate with simpler /readonly option. (/noupdate is
supported for backwards compatibility but no longer documented.)

Better checking of the compatibility between per_* options and the
ACL in which the ratelimit condition appears.

Better handling of the start of a burst of email and of very low-rate
clients.

The new /count= option generalizes the per_byte and per_rcpt options.

The new /unique= option is a rather groovy use for a Bloom filter.