Limit MQTT packet size for MQTT 3.x connections

- Enforce the configured packet size limit locally for MQTT 3.1/3.1.1.
- Close the connection if the broker sends a packet exceeding the limit.

MQTT 3.1 and 3.1.1 have no way to tell the broker the maximum packet
size the client accepts, so the limit can only be applied on the
client side. This is helpful for clients on embedded devices, where
memory is limited and a very large packet from the broker could
exhaust it.

[ChangeLog][QMqttClient] The maximumPacketSize connection property is
now also enforced for MQTT 3.1 and 3.1.1 connections. Previously it was
only honored for MQTT 5.0. The client now closes the connection if the
broker sends a packet larger than this limit. This helps clients on
embedded devices to protect their limited memory. The default behavior
is unchanged: if no limit is set, only the protocol maximum of 256 MB
applies.

Change-Id: I26625325f490e2396fa493005c1f322253007a33
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>