Federated and Federatex cannot be used with ROR scans

Federated::position() and Federatex::position() is storing in 'ref' a
pointer into a local result set buffer. This means that one cannot
compare 'ref' from different handler instances to see if they point to the
same physical record.

This bug caused federated.federatedx to return wrong results when the
optimizer tried to use index_merge to resolve some queries.

Fixed by introducing table flag HA_NON_COMPARABLE_ROWID and using this
with the above handlers.

Todo:
- Fix multi_delete(), multi_update and read_records() to use primary key
  instead of 'ref' if case HA_NON_COMPARABLE_ROWID is set. The current
  code only works if we have only one range (like table scan) for the
  tables that will be updated in the second pass.
- Enable DBUG_ASSERT() in ha_federated::cmp_ref() and
  ha_federatedx::cmp_ref().