add PHP_GRPC_FREE_CLASS_OBJECT macro

src/php/ext/grpc/call.c

src/php/ext/grpc/call_credentials.c

src/php/ext/grpc/channel.c

src/php/ext/grpc/channel_credentials.c

src/php/ext/grpc/php7_wrapper.h

src/php/ext/grpc/server.c

src/php/ext/grpc/server_credentials.c

src/php/ext/grpc/timeval.c