add singleton setting for open layershell

[Decodetalkers]

now there is a problem that when I request a new layershell, in very short time if I request it again. the id has not be set, so in client side I cannot know that the layershell is ready for being created, so the workflow will be better control by lib. lib should check if the enum is for singleton. if a new request is in pending, then new request should not be accepted