fix issues/873

[alessandroAmedei]

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fix #909 #873 #1005

Fixing the java docs of the method
org.eclipse.paho.mqttv5.client.setSessionExpiryInterval(long)

Without calling this method with parameters greater than 0 or different from null,
once the Network Connetion is closed, the broker will end immediatly the session even if the client
is using cleanSession = false.

[alessandroAmedei]

To make sure that session is never deleted by broker:

connOpts.setSessionExpiryInterval((long) Integer.MAX_VALUE);