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
* Use python executable directly
* python2 is not guaranteed to exist
* scripts aren't directly executable on Windows
* Add Firestore v1 protos
* Point cmake at Firestore v1 protos
* Regenerate protobuf-related sources
* Make local protos refer to v1 protos
* fixup! Regenerate protobuf-related sources
* Remove v1beta1 protos
* s/v1beta1/v1/g in source.
* s/v1beta1/v1/ in the Xcode project
* Remove stale bug comments.
This was fixed by adding an explicit FieldPath API rather than exposing
escaping to the end user.
Gil