THRIFT-3112 [Java] AsyncMethodCallback should be typed in generated AsyncIface

The parametrization brings the existing actual parametrization with
client call implementation objects to the fore and so this change
also fixes that parametrization to be a simple parametrization over
the return type as is done in the server-side AsyncProcessor code.

NB: This is a breaking change in both generated code and the client
libs.

This closes #840
7 files changed