WARNING: this commit changes the DB representation; you need to alter the

WARNING: this commit changes the DB representation; you need to alter the
         DB by hand after this commit

- h_position: varchar(255) -> varchar(62)
- h_sort:     varchar(255) -> varchar(6)
- several indexes have been redesigned from scratch.
  Motivation: MySql is not able to exploit two indexes (on columns c1 and c2)
  in a query of type "select ... from table where c1 = x and c2 = y".
  Only one of the two indexes is exploited; the other test is done on each line.
  My commit introduces a single index on c1 AND c2 in place of two indexes.

WARNING: it is sure that a few queries (e.g. match) are greatly optimized by
this commit. On the contrary I ignore if the performance of other queries drops.
Moreover, I have not optimized yet the indexes over the hits and count tables.