Mobile/Fennec/HistoryExpiration

We're looking for ways to both reduce memory use and improve the speed of our queries. Bug 744961 added some history expiration that occurs in two different situations:

History

• On every Pause of the app (i.e. even when just opening the Settings page) we throw away any history that is
  • Older than one month since it was last modified/visited
  • We always leave at least 2000 entries, sorted by frecency. i.e. if you have less than 2000 visits we dont delete anything

• When we receive the ACTION_DEVICE_STORAGE_LOW notification from android we
  • Delete history entries with the least frecency score, leave 500 entries

Favicons

No plans to evict these yet. We should make sure we're evicting them using the same strategy shown above (i.e. whenever we expire history)

Thumbnails

We're not doing anything yet. But eventually I'd like to only keep a few of these around (10?) ever.