sql: add collations and DESC for ephemeral table

 - Added ability to specify collations for ephemeral tables.
 - Enabled DESC sorting order. It is implemented as ASC, but rows are
   taken from sorting table in reversed order. Currently, it is
   impossbile to specify more than one sorting order for all columns in
   ORDER BY clause due to incapability of iterating through different
   orders in Tarantool.  As soon as this feature is added to Tarantool, it
   will be implemeted in SQL.
 - Fixed code generation for adding rows to ephemeral table while
   coroutine yields.
 - Temporary disabled tests which used different sorting orders in one
   query.

Part of #2680