send stored connections to ui in batches

Batches sending the "stored connections" from the addon-side simple-storage to the UI when the tab is opened.

All parameters (batch size, setTimeout delay) are chosen fairly arbitrarily, with some influence from [this article on yielding with JS timers](http://answers.oreilly.com/topic/1506-yielding-with-javascript-timers/).

This works well on my Linux machine, but I ran into some issues testing it with my "normal profile". After digging around, it seems like there might be a bug elsewhere with the way we're deleting things to stay under quota (which I will try to look into more, later). In the meantime, I'm interested in hearing how smooth, fast, and janky-or-not things are on other platforms (since my Firefox on Linux doesn't have GPU support, the graph rendering is invariably janky).

Should close #572.