Bug 861955 getFavicon async method in io/data

Added async `getFavicon` for `io/data`, added deprecation message for previously sync version. 

* The favicon service only stores visited sites in the cache, so the get will only return visited sites, hence the tests opening up the site and waiting -- been playing around with the deprecated sync methods of setting a specific path/binary data so we don't have to play with timers, so trying that out so we can avoid timers altogether