Also properly including our headers from public headers using <>

As a no-op for the time being.

Introduced grpc_byte_buffer_init instead. It's now the user's responsibility to
manage memory.

Still TODO:
- a way to plug a metadata processing (somewhat elsewhere but did not
  one to overload this already large PR).
- plug-in the auth context on the client side.
- Better end to end testing.

Port fling_test to it.

This will be used to:
- port remaining tests to Windows
- enable testing what happens when servers or clients mysteriously disappear