This commmit changes the command-line and Bridge line arguments to take
a comma-separated list of front domains. The change is backwards
compatible with old Bridge and ClientTransportPlugin lines. At
rendezvous time, a front domain will be randomly chosen from the list.
This continues to asserts the known version while decoding. The client
will only ever generate the latest version while encoding and if the
response needs to change, the impetus will be a new feature, set in the
deserialized request, which can be used as a distinguisher.
Makes BrokerChannel abstract over a rendezvousMethod. BrokerChannel
itself is responsible for keepLocalAddresses and the NAT type state, as
well as encoding and decoding client poll messages. rendezvousMethod is
only responsible for delivery of encoded messages.
