Add user_update_cache function that provides a quick way to update a cached field without reloading the data from the DB. This saves a query on each page atm for last_visited value.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@4740 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9