Add connected event handler

[UtechtDustin]

This PR will add a new event handler list for an "connected"-event (ref #143).
I also had to add an duplicate check for the subscriptions, to prevent duplicated execution of the subscriptions callbacks.
I'm not sure if this should be the goal, maybe it could make more sense to add option for it or maybe clear the subscriptions if the connection is lost.

@Namoshek Do you have an idea for the duplicated subscription issue ?

[Namoshek]

@UtechtDustin Thank you for the PR!

Looking at the SUBSCRIBE/SUBACK logic, I think that re-subscribing is only possible if the broker accepts it and therefore it should be safe to replace the old subscription with the new one. That's what I've done now.

I also extended the new event handler by passing an $isAutoReconnect flag. This should give you an additional hint whether it is safe to resubscribe or not.

Additionally, I've added some sweet integration tests to make sure this stuff actually works.

Please let me know whether that looks good to you. 👍

[UtechtDustin]

Looks good for me, thank you for the quick response :)

[Namoshek]

Not sure why those builds all failed with a false-positive, but anyway, the feature is release as v1.8.0. 👍