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
ok so let me summarize what this does so I get it right:

The goal of the list is to have the 10 last Simple Push URLs for a given user.

* the list size varies from 0 to 10
* the list contains unique URLs
* when an URL is added, any previous occurence of this URL is removed  

I like what you did. One optimization : an lpush() call returns the new size of the list,
so you can avoid an extra ltrim() call if the size is <= 10