MqttSubscriptionClient: connection lost (issue #22)

[olga-dorogan]

Issue #22 : MqttSubscriptionClient: connection lost

Description of changes:
Added creation of HashSet with subscription objects just before iteration in onError() callback in subscribe() method.
As iteration over getSubscriptionObjects() result is not synchronized with other methods that can changesubscriptionsByTopic field value, modification of topic subscribers while onError() callback is in progress can cause ConcurrentModificationException. As subscriber (SubscriptionObject), for whom onError() should trigger onFailure() on listeners, already has been added to the set, the simplest way is to create copy of getSubscriptionObjects() result and iterate over it.
I tried to use 1.1.0 version of org.eclipse.paho:org.eclipse.paho.client.mqttv3 with the latest versions of other libraries as suggested in the issue comments, but in this case subscription called OnCompleted() right after it had been started and didn't send any successfull responses.

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[scb01]

@olga-dorogan
Thank you for your help in making this SDK better. The changes look great and I will merge them in.