best_extension_by_limited_search() assumes that tables should be sorted
according to size to be able to quickly disregard bad plans. However the
current usage of swap_variables() will change the table order to a not
sorted one for the next recursive call. This breaks the assumtion and
causes performance issues when using many tables (we have to examine
many more plans).

This patch fixes this by ensuring that the original table order is kept
for the not yet used tables when best_extension_by_limited_search() is
called.

This was done by always calling swap_variables() for each table and
restoring the original table order at exit.

Some test changed:
- In a majority of the test the change was that two "identical tables"
  where swapped and the optimzer is now using the first/smaller table
- In few test the table order was changed. The new plan looks identical
  or slighly better than the original.