4.0.0: Unwrap unions and improve RPC API.

+ Unions are now represented more naturally without a wrapping object. The
  previous wrapped union type is still available to support ambiguous (less
  common) combinations of branches.
+ Protocol emitters and listeners now expose a lower level API which supports
  exchanging custom headers, running middleware, and more. Implementing custom
  transport encodings is now also much simpler.

Other changes:

+ Types can now export their schemas with defaults and other attributes (e.g.
  logical type information).
+ The default stateful RPC implementation now uses a Netty-compatible encoder
  (which will work out of the box with the Avro's Java implementation).