firebase-ios-sdk

gRPC: replace Objective-C implementation with the new C++ implementation (#2068)

* Revert "Revert "gRPC: replace Objective-C implementation with the new C++ implementation (#1968)" (#2030)"

This reverts commit ea567dca47e44bce27c3fe66862aad90371ecaec.

* gRPC: fix bugs found during testing (#2039)

All in `GrpcStream`:
* make sure there are no pending completions when "finish" operation is enqueued;
* always finish gRPC calls;
* when connectivity changes, reset not just the calls, but the underlying channel as well.

* Revert "Partially revert "Update CHANGELOG for Firestore v0.14.0 (#2025)" (#2031)"

This reverts commit b2437b80f61b031496c0c9bd7a1cc871b9a3a6b9.

* Update changelog for v0.15.0.

* gRPC: add gRPC wrapper classes to CMake build (#2015)

* add a C++ equivalent of `FIRFirestoreVersionString`. This eliminates the only Objective-C dependency of `grpc_connection.mm` and allows making it a pure C++ file;
* open-source `binary_to_array.py` script and use it in the build to embed `roots.pem` certificate file from gRPC into the Firestore binary. The embedded char array is then used to load certificate at runtime.

* Revert local change -- do not enable sanitizers by default