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>

linux/shim/service.go

@@ -3,13 +3,11 @@
 package shim
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"sync"
 
-	"google.golang.org/grpc"
-	"google.golang.org/grpc/codes"
-
 	"github.com/containerd/console"
 	eventstypes "github.com/containerd/containerd/api/events"
 	"github.com/containerd/containerd/api/types/task"
@@ -27,7 +25,8 @@ import (
 	ptypes "github.com/gogo/protobuf/types"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 var empty = &ptypes.Empty{}
@@ -167,7 +166,7 @@ func (s *Service) DeleteProcess(ctx context.Context, r *shimapi.DeleteProcessReq
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	if r.ID == s.id {
-		return nil, grpc.Errorf(codes.InvalidArgument, "cannot delete init process with DeleteProcess")
+		return nil, status.Errorf(codes.InvalidArgument, "cannot delete init process with DeleteProcess")
 	}
 	p := s.processes[r.ID]
 	if p == nil {