Track outstanding advance jobs to prevent double-scheduling

[narinearraul]

mkistler commented on Dec 18, 2016
The SDK should track outstanding advance jobs to determine when it needs to schedule an advance as part of processing a new listen. Currently the SDK checks for an existing listener, and skips scheduling an advance if there is already a listener on the same key. But this could result in multiple advance jobs for the same key if the app unlistens and then quickly relistens on the same key, the listen will not see an outstanding listener, but the advance job from the previous listen may still be active.

A fix for this issue is already implemented in the JS SDK and we may want to use the same approach here.