Respect channelMax

Limit number of channels to channelMax as negotiated with the server.
Try to find an available channel id, even if it means to wrap around.
Return ErrChannelMax if no available channel id can be found.

Addresses #115
Then lets make sure callers of `DialConfig` can still use the zero value for `Config.Channels` and the client will do what is expected - use as many bits as possible for open channels.