Hardcoded table column names in the ClosureTable model

[franzose]

Originally reported by @devig in #234.

ClosureTable::selectRowsToInsert() uses hardcoded table column names:

 $select = "
     SELECT tbl.{$ancestor} AS ancestor, ? AS descendant, tbl.{$depth}+1 AS depth
     FROM {$table} AS tbl
     WHERE tbl.{$descendant} = ?
     UNION ALL
     SELECT ? AS ancestor, ? AS descendant, 0 AS depth
 ";

This will break SQL queries in case of non-standard column names are used. The above should be changed to the below:

$select = "
    SELECT tbl.{$ancestor} AS {$ancestor}, ? AS {$descendant}, tbl.{$depth}+1 AS {$depth}
    FROM {$table} AS tbl
    WHERE tbl.{$descendant} = ?
    UNION ALL
    SELECT ? AS {$ancestor}, ? AS {$descendant}, 0 AS {$depth}
";