vendor: update grpc and dependent package. (#89)

* vendor: update grpc package to v1.18.0.

In order to use grpc package in v1.18.0, the following dependent vendor
packages are updated:

- github.com/golang/protobuf/proto
- golang.org/x/sys/unix
- google.golang.org/grpc
- google.golang.org/grpc/balancer
- google.golang.org/grpc/credentials
- google.golang.org/grpc/internal
- google.golang.org/grpc/stats
- google.golang.org/grpc/resolver

The following package are removed because grpc v1.18.0 uses context package in
standard library.

- golang.org/x/net/context
- golang.org/x/net/context/ctxhttp

* Use context package in standard library.

* Add back packages under golang.org/x/net/context.

Some vendored package still use packages under golang.org/x/net/context, the
following two packages are added back and updated to latest version:

- golang.org/x/net/context
- golang.org/x/net/context/ctxhttp

Author: StevenYCChou

bench/main.go

@@ -13,6 +13,7 @@ limitations under the License.
 package main
 
 import (
+	"context"
 	"flag"
 	"fmt"
 	"io/ioutil"
@@ -25,7 +26,6 @@ import (
 	"github.com/golang/protobuf/ptypes/empty"
 	grpc_prometheus "github.com/grpc-ecosystem/go-grpc-prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
-	"golang.org/x/net/context"
 	monitoring_pb "google.golang.org/genproto/googleapis/monitoring/v3"
 	"google.golang.org/grpc"
 )

stackdriver/test_server.go

@@ -13,8 +13,9 @@ limitations under the License.
 package stackdriver
 
 import (
+	"context"
+
 	empty_pb "github.com/golang/protobuf/ptypes/empty"
-	context "golang.org/x/net/context"
 	metric_pb "google.golang.org/genproto/googleapis/api/metric"
 	monitoredres_pb "google.golang.org/genproto/googleapis/api/monitoredres"
 	monitoring_pb "google.golang.org/genproto/googleapis/monitoring/v3"