mqtt client in unix, connectionLost is not invoked on network outage

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- [ x ] Bug exists Release Version 1.2.0 ( Master Branch)
- [ x ] Bug exists in MQTTv3 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)
- [ ] Bug exists in MQTTv5 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)

If this is a bug regarding the Android Service, please raise the bug here instead: https://github.com/eclipse/paho.mqtt.android/issues/new

Steps:

1. Start broker service on a machine.
2. Start java client using this library on unix system.
3. remove the network between them. For this, I created a firewall rule for inbound on port 1883 and denied service. This I did on the machine where broker is running.

Expected:
1. the connectionLost() method is invoked. At least after the keepalive interval is over.

Actual:

connectionLost() is not invoked.

I have a probable fix also. I will raise a PR and update.