TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods

* Reference discussion: https://github.com/grpc/grpc/pull/4108#discussion-diff-44494393

For local running one can still tests AllTests, but for C.I. one can
test the 4 of them in parallel (and, importantly, deactivate the remote
interop tests until we fix them on Travis; while keeping the rest).

I had to make the remote tests a subclass of an abstract InteropTests
(like InteropTestsLocalSSL and InteropTestsLocalCleartext), and then
deactivate the abstract InteropTests on the 4 schemes that use it.

Eventually all samples will be tested.

Still needs implementing format_as_block, and switching input and output
to use sys.stdin.readline() and sys.stdout.write(line).

Corrected compiler warning for effectively pure virtual ObjC methods.

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.

The UnaryCall interop test is now adequately specified in
doc/interop-test-descriptions.md.

With both raw and codegen’d library