[EventHubs] separate Avro API from Buffered Producer changes

[swathipil]

For May 2020 release of EH, we need to do the 5.9.0 GA the Avro API changes. Currently, the 5.9.0b3 includes BufferedProducer, which we don't want to GA yet.
Questions:

1. What will be the new version for the BufferedProducer? --> 5.10.0b1

We need to separate these, by either:

1. Making all BufferedProducer public API private, but keeping actual code changes on main.
2. Moving all BufferedProducer changes to a feature branch. Then, removing all BufferedProducer code on main so that only Avro updates are on main. (Probably more copy/paste work..)

Probably prefer 2, since we want to do a few more beta releases for the BufferedProducer.