Implement async_void in alterl bindings

Summary:
  - reply_type for async void functions is now async_void instead of
    the empty struct definition
  - async void functions should return "ok". otherwise the processor
    will crash and the connection will be killed. Is this behaviour
    expected?

Test Plan: tested using testAsync() in ThriftTest. Didn't used to work
           but works now


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@666415 13f79535-47bb-0310-9956-ffa450edef68
3 files changed