The GRPCCall import must be in the header because it defines a protocol
that GRPCRequestHeaders uses.

The GRPCCall.h file is already correctly imported in the
GRPCRequestHeaders.m implementation file. This header shouldn’t need to
import GRPCCall and should specify a class forward declaration for
reference.

This is very much unsafe as the string_ref could point on a stack
variable of the callee.

the channeld->content_type->key is already consumed by the preceding if.

This reverts commit 784db97a.

Fixes #3203

We entered a live lock race between the promotion callback and pollers,
which was only resolved when the background executor got lucky enough to
pick up the async work.

Plus test tweaks and final touches to round robin policy

The old package directory handling was stale in grpcio's setup.py
command support module and docgen had a typo.

- We cannot use call_data after the final callback as this one may
  trigger the deletion of the call object (hence the call_data).