clear connection tables when transport is changed

[catmando]

its possible to get pretty messed up when changing between transports, if you have data left in the connection tables.

Best to track which transport type is being used in the tables, and then clearing them out if it doesn't match the current type.

The key way things get messed up is that simple poller and pusher use the refresh_at field, while action_cable does not. So if you (for example) move from simple poller to action_cable, and you have some connections left in the table, things blow up. Best to put some safety around the refresh_at, so it gets ignored if you are in action_cable, instead of dying.