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