Changes to Places memory consumption, expiration and performances - More sleep, less work.

As part of the Memshrink effort, we reduced the SQLite page cache used by Places. This is not a leak, the page cache is actively used by SQLite to improve performances by reducing disk I/O. Originally Places was allowed to use a large cache (maximum 6% of physical memory, per connection), to ensure responsiveness of the awesome bar.Starting from the 20110901 Nigthly, the following changes have b

[taninsw]

最新7日間の履歴はexpireしないようになる。

[Rockridge]

Fx9：Placesデータベースは、これまで最大で物理メモリの6%をキャッシュに使用していた。これが物理メモリの2%とデータベースサイズの半分との間の最小値（ただし5MB以上）へと改められた。