#693: DataPublisher created from async function should start lazily

[khlopko]

Fix for the issue: #693.

Future is generally better approach than PassthroughSubject when you need to execute something once (e.g. network requests). What was failing in the original solution by the issue author is that Future starts execution right after creation as well, and needs to be wrapped in Deferred in order to start only after subscribing to it.

[kean]

Works as advertised, thanks, @khlopko. I appreciate you taking time to address it.