Allow only one HTTP poll at a time per extension

Without this change, long-running poll requests (such as those resulting in a timeout) can cause arbitrarily long request queues, eventually leading to spontaneous termination of the application.
To clarify "seems like it shouldn't make a difference": the only situation in which it would make a difference is if two different extensions were polling the same URL, which shouldn't ever happen. Even if it does happen, we'd just have up to a handful of polls queued... still less than the tens of thousands that I was sometimes seeing without this change.