- Performance improvement: made 'sid' the primary key of the sessions table.
  That should improve performance of session handling as well improve
  performance of the "Who's online"-block.  Drupal.org's sessions table
  contains appr. 40.000 sessions on a slow day and rendering the "Who's
  online"-block became a performance bottleneck.

  This change has yet to be tested on a busy site so things might go wrong.