Ability to set different connection limits per vhost user group

The ability to set per-user limitations is necessary in a shared infrastructure environment where users are given different quotas of messaging.

With these settings a vhost may be configured so that users in a vhost group may receive different limits than the vhost has overall. The original motivation was to allow administrators more connections per user and per host than everyone else but the limits may be applied to raise or lower the bounds for any group.