(MQTT-11) Length of client Identifier

[Roger Light <roger@atchoo.org>]

The client ID length limit is one of the points that has come up
frequently when people have been talking to me about MQTT.

There is no intrinsic reason within the protocol to limit it to 23
characters. I would be keen on increasing the limit.

I've carried out a survey of existing MQTT implementations

Servers:

mosquitto, activemq, rabbitmq, apollo, rsmb, mqtt.js

Client libraries:

libmosquitto, python-mosquitto, paho c, paho java, paho javascript, paho lua.

Of these, only rabbitmq on the server side and paho java and
javascript implement the 23 character client id limit.

Even a minor increase in length to 32 bytes would be very helpful.