sql: do not bless tuples

Tuple blessing is needed generaly for Lua. Bless references a
tuple before returning to be able to unref it, when a next result
is returned, and the previous one is already gone into Lua, or
another public API.

For internal usage it is not needed to bless a tuple:
- to just get a tuple field you can skip even simple ref, because
  tuple_field_...() does not yield, and the tuple can not be
  deleted during it;
- for internal SQL iterators you simply reference a new tuple,
  and unref a previous for youself, with no bless.