set default http idle connection per host

possible fix https://github.com/chrislusf/seaweedfs/issues/915

weed/util/grpc_client_server.go

@@ -3,6 +3,7 @@ package util
 import (
 	"context"
 	"fmt"
+	"net/http"
 	"strconv"
 	"strings"
 	"sync"
@@ -18,6 +19,10 @@ var (
 	grpcClientsLock sync.Mutex
 )
 
+func init(){
+	http.DefaultTransport.(*http.Transport).MaxIdleConnsPerHost = 100
+}
+
 func NewGrpcServer(opts ...grpc.ServerOption) *grpc.Server {
 	var options []grpc.ServerOption
 	options = append(options, grpc.KeepaliveParams(keepalive.ServerParameters{