<rdar://problem/5336372>
        Icon database uses too much memory
        
        XRaying Safari startup memory consumption revealed that icon database is eating quite
        a bit of RAM if Icon.db is large (which it probably is if it has been in use for a while, 
        mine used for getting figures below was 2.6MB). 
        
        Note that the wins are less impressive with smaller Icon.db.
        
        This patch addresses three separate issues
        
        - SQLite fails to free the memory used by temporary tables. Icon database uses a temporary table
          on startup for pruning unused page urls. This wastes around 1MB. Addressed by rewriting
          pruning so it does not need a temporary table. The new method is also quite a bit faster speeding
          up Safari launch time by around 100ms
        - SQLite has it's own memory cache limited by default to 3MB. Icon database does not really need that much.
          Dropped the cache size to 300kB saving ~1MB on startup. 
          Smaller cache slows down startup by ~30ms (more than compensated by faster pruning above)
        - Don't populate m_pageURLToIconURLMap with all urls from database on startup, instead let it get populated
          when urls are accessed (user opens history menu for example). This shouldn't have any real performance impact 
          as the accesses are icon loads that need to hit the database anyway. This saves ~700kB.
          
        All in all with this Icon.db these changes reduce allocated memory by around 2.7MB on startup. Release build
        Safari RPRVT (empty start page) goes from 12.4MB to 10.4MB (TCMalloc pooling probably explaining why the win 
        looks bit smaller here).

        * loader/icon/IconDatabase.cpp:
        (WebCore::IconDatabase::IconDatabase):
        (WebCore::IconDatabase::open):
        (WebCore::IconDatabase::deleteAllPreparedStatements):
        (WebCore::IconDatabase::retainIconForPageURL):
        (WebCore::IconDatabase::releaseIconForPageURL):
        (WebCore::IconDatabase::establishIconIDForIconURL):
        (WebCore::IconDatabase::pruneUnretainedIconsOnStartup):
        * loader/icon/IconDatabase.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@24364 268f45cc-cd09-0410-ab3c-d52691b4dbfc