agent: Support more than one channel in the cockpit-agent

Fixes #256
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Send multiple channels for same host/user to same agent

Now that the agent has support for multiple channels we can
send multiple channels for the same host (and user, in case
it's different) down the same transport and to the same agent.

The webservice times out a agent after 30 seconds of there
being no channel around on it.

Fixes #256
Closes #272
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>