This unblocks nanopb changes proposed in #4264.
This upgrades everything in the gRPC constellation of dependencies to match gRPC at 1.28:
* gRPC to 1.28.0 (note that gRPC 1.28.1, while released, is a fix for just the python binding and there's no gRPC-C++ CocoaPod at that version)
* Abseil to 20200225 (as of gRPC 1.27 this is now a gRPC dependency too; there's a later patch but gRPC doesn't use it yet)
* Protobuf C++ to 3.11.4 (only used for testing in Firestore)
* C-Ares to 1.15.0
* BoringSSL to match (no change between gRPC 1.27.0 and 1.28.0)
Also, this disables CFStream-based transport on Apple platforms. This works around b/133182964, wherein CFStream will occasionally fail to raise a has-bytes-available events, causing Firestore to appear to hang.
* Upgrade gRPC to 1.24.3
* Upgrade C++ Protobuf to 3.9.2
Disable -Wunreachable-code in protobuf-generated code
* Regenerate C++ protos with protobuf 3.9.2
* Generate objc protos with Protobuf 3.9.2
Previously, the tests would compare serialization results against a
precomputed (via protoc) array of bytes. Now they serialize via our
nanopb based class and deserialize via libprotobuf (and vice versa) and
then ensure the result is the same as the input
Gil