Add a GrpcLibrary class to wrap grpc_init and grpc_shutdown and convert all the tests to init/shutdown free.

So the root of the repository doesn’t have 2 podspecs.
All four libraries are subspecs now.
Adjusts the generated code to account for that.

The stub is manually adapted:

- to the evolved runtime,
- as if generated with insertion points.

Command:
protoc --plugin=/usr/local/bin/protoc-gen-objc test.proto
messages.proto empty.proto
--proto_path=/Users/jcanizales/git/protobuf-objc/src/compiler
--proto_path=. --objc_out="./"