vinyl: run iterator: refactor seek method

A few changes to make the function more straightforward:

 - Move bloom checking and LSN filtering out of 'do_seek' helper. Make
   the helper do just one simple task - lookup the first one in a series
   of statements matching the given search criteria.
 - Fold iterator type and key substitution in 'seek' method, similarly
   to how we did it for other iterators.
 - Cleanup EQ checks. Use the original iterator type and key where
   appropriate to remove extra checks in calling methods. Don't check EQ
   in 'seek' method in case it was checked by 'do_seek'.
 - Add some comments.