lnd: fix build error for go 1.6.3, use experimental context import

This commit fixes a build error for go 1.6.3. Before go 1.7, the
context package was under the experimental go packages, therefore to
maintain compatibility across our currently supported versions, we must
use the ‘/x/net’ import path.

lnd.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"context"
 	"encoding/hex"
 	"fmt"
 	"io/ioutil"
@@ -13,6 +12,8 @@ import (
 	"runtime"
 	"strconv"
 
+	"golang.org/x/net/context"
+
 	"google.golang.org/grpc"
 
 	proxy "github.com/grpc-ecosystem/grpc-gateway/runtime"