storage-users: Add config option for graceful shutdown timeout

Add STORAGE_USERS_GRACEFUL_SHUTDOWN_TIMEOUT setting to allow a graceful
shutdown of the storage-users service. This currently only applicable
when running storage-user as a sepearate service.

Setting STORAGE_USERS_GRACEFUL_SHUTDOWN_TIMEOUT to a non-zero value
gives the storage-users service a chance to cleanly shutdown and finish
any in progess tasks (e.g. metadata propagation) before exiting.

Partial-Fix: #6602

services/storage-users/pkg/config/defaults/defaultconfig.go

@@ -41,12 +41,13 @@ func DefaultConfig() *config.Config {
 		Service: config.Service{
 			Name: "storage-users",
 		},
-		Reva:             shared.DefaultRevaConfig(),
-		DataServerURL:    "http://localhost:9158/data",
-		DataGatewayURL:   "https://localhost:9200/data",
-		TransferExpires:  86400,
-		UploadExpiration: 24 * 60 * 60,
-		Driver:           "ocis",
+		Reva:                    shared.DefaultRevaConfig(),
+		DataServerURL:           "http://localhost:9158/data",
+		DataGatewayURL:          "https://localhost:9200/data",
+		TransferExpires:         86400,
+		UploadExpiration:        24 * 60 * 60,
+		GracefulShutdownTimeout: 30,
+		Driver:                  "ocis",
 		Drivers: config.Drivers{
 			OwnCloudSQL: config.OwnCloudSQLDriver{
 				Root:                  filepath.Join(defaults.BaseDataPath(), "storage", "owncloud"),