linux/shim: reduce memory overhead by using ttrpc

By replacing grpc with ttrpc, we can reduce total memory runtime
requirements and binary size. With minimal code changes, the shim can
now be controlled by the much lightweight protocol, reducing the total
memory required per container.

When reviewing this change, take particular notice of the generated shim
code.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/containerd-shim/shim_unix.go

@@ -6,10 +6,9 @@ import (
 	"os"
 	"os/signal"
 
-	"google.golang.org/grpc"
-
 	"github.com/containerd/containerd/reaper"
 	runc "github.com/containerd/go-runc"
+	"github.com/stevvooe/ttrpc"
 )
 
 // setupSignals creates a new signal handler for all signals and sets the shim as a
@@ -23,6 +22,6 @@ func setupSignals() (chan os.Signal, error) {
 	return signals, nil
 }
 
-func newServer() *grpc.Server {
-	return grpc.NewServer()
+func newServer() *ttrpc.Server {
+	return ttrpc.NewServer()
 }