Refactor shim terminal and io handling

This also finishes the service implementation of the shim behind GRPC

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/containerd-shim/main.go

@@ -52,7 +52,9 @@ func main() {
 		}
 		var (
 			server = grpc.NewServer()
-			sv     = &service{}
+			sv     = &service{
+				processes: make(map[int]process),
+			}
 		)
 		shim.RegisterShimServiceServer(server, sv)
 		l, err := utils.CreateUnixSocket("shim.sock")