Add a GrpcLibraryInitializer to client_context.cc

It's feasible that a program be written/linked such that it only use
ClientContext from grpc++, which could end up with the other instances
of GrpcLibraryInitializer not ending up in the final binary.  Add a
GrpcLibraryInitializer to client_context.cc to ensure that the library
is initialized.  The primary side-effect of the library not being
initialized when only using a ClientContext is that the destructor for
ClientContext indirectly ends up trying to call through
g_core_codegen_interface when destructing its metadata, which is null.