Client stops working after subscribing to a topic with lots of retained messages

- [x] Bug exists Release Version 1.2.5 ( Master Branch)
- [x] Bug exists in MQTTv3 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)
- [x] Bug exists in MQTTv5 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)

We have a MQTT broker holding several thousands of topics with retained messages. If we try to connect and subscribe (wildcard) to a parent topic the paho client stops working.
Depending on the length of the topic and the message and the network connection, the client stops working after a number of retained messages.

With Mosquitto MQTT broker somewhere in the internet it takes less than 2000 messages.
With Mosquitto MQTT broker on localhost, less then 40000 messages.

I tried to find the cause, but ended up somewhere in the depths of the CommsReceiver.

Reproduce the failure with a Mosquitto broker, a wildcard subscription, and many retained messages using the 1.2.5 or 1.2.6-SNAPSHOT client. Start by tracing the CommsReceiver while varying topic and message lengths and network location. Done means the client remains responsive while receiving the retained messages.

Written by the indexing model from the issue text.