Bug 985387 — Store multiple simple push URLs

https://bugzilla.mozilla.org/show_bug.cgi?id=985387

This provides a mechanism to handle multiple push URLs.

The problem with this approach is that we don't have any way to expire individual members of the set
I am a bit confused. you are removing the first element, adding one back, then triming to 10 elements. 

Why do we need to trim if we are replacing an element ? are there other places in the code that makes that list grows?