Nit: use time.Durations instead of constants in seconds. (#7438)

Nit: use `time.Durations` instead of constants in seconds. Makes for slightly more readable code.

weed/server/constants/volume.go

@@ -1,5 +1,7 @@
 package constants
 
+import "time"
+
 const (
-	VolumePulseSeconds = 5
+	VolumePulsePeriod = 5 * time.Second
 )

weed/server/volume_grpc_client_to_master.go

@@ -68,7 +68,7 @@ func (vs *VolumeServer) heartbeat() {
 				master = newLeader
 			}
 			vs.store.MasterAddress = master
-			newLeader, err = vs.doHeartbeatWithRetry(master, grpcDialOption, time.Duration(vs.pulseSeconds)*time.Second, duplicateRetryCount)
+			newLeader, err = vs.doHeartbeatWithRetry(master, grpcDialOption, vs.pulsePeriod, duplicateRetryCount)
 			if err != nil {
 				glog.V(0).Infof("heartbeat to %s error: %v", master, err)
 
@@ -81,7 +81,7 @@ func (vs *VolumeServer) heartbeat() {
 				} else {
 					// Regular error, reset duplicate retry count
 					duplicateRetryCount = 0
-					time.Sleep(time.Duration(vs.pulseSeconds) * time.Second)
+					time.Sleep(vs.pulsePeriod)
 				}
 
 				newLeader = ""

weed/server/volume_server.go

@@ -35,7 +35,7 @@ type VolumeServer struct {
 	SeedMasterNodes []pb.ServerAddress
 	whiteList       []string
 	currentMaster   pb.ServerAddress
-	pulseSeconds    int
+	pulsePeriod     time.Duration
 	dataCenter      string
 	rack            string
 	store           *storage.Store
@@ -59,7 +59,7 @@ func NewVolumeServer(adminMux, publicMux *http.ServeMux, ip string,
 	folders []string, maxCounts []int32, minFreeSpaces []util.MinFreeSpace, diskTypes []types.DiskType,
 	idxFolder string,
 	needleMapKind storage.NeedleMapKind,
-	masterNodes []pb.ServerAddress, pulseSeconds int,
+	masterNodes []pb.ServerAddress, pulsePeriod time.Duration,
 	dataCenter string, rack string,
 	whiteList []string,
 	fixJpgOrientation bool,
@@ -86,7 +86,7 @@ func NewVolumeServer(adminMux, publicMux *http.ServeMux, ip string,
 	readExpiresAfterSec := v.GetInt("jwt.signing.read.expires_after_seconds")
 
 	vs := &VolumeServer{
-		pulseSeconds:                  pulseSeconds,
+		pulsePeriod:                   pulsePeriod,
 		dataCenter:                    dataCenter,
 		rack:                          rack,
 		needleMapKind:                 needleMapKind,