Check implementation of broadcast policies for performance issues

Typical system running under light load may have several hundred active connections.
Lets make sure the broadcast policy mechanism does not do stupid things like iterate over all connections, and that ActiveRecord is properly caching repeated calls to the Connection.active query.

Start by locating the broadcast policy implementation and every call to the ActiveRecord Connection.active query. Measure or inspect the behavior under many active connections, then verify that repeated calls are cached and that the policy mechanism does not iterate over all connections unnecessarily.

Written by the indexing model from the issue text.