Add a run method to destinations

[alexanderkirtzel]

Is your feature request related to a problem? Please describe.
Right now a destination only has an init and a push method that can be called. There is no chance of telling a destination, that a new run started.

Describe the solution you'd like
Add a third, optional run interface that gets called on each new run.

Describe alternatives you've considered
Using a page view event is not the same as a real run. We need this explicit one.

[alexanderkirtzel]

We won't add a run method to a destination. Instead the instance's config aka runState is now available and will be passed as a fourth argument. One can check for the round number and deal with it. This keeps the code smaller. Might come back to this one when there are more common use cases.

[alexanderkirtzel]

v.1.6.3