MDEV-14907 FEDERATEDX doesn't respect DISTINCT

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().

sql/handler.h

@@ -329,7 +329,17 @@ enum enum_alter_inplace_result {
 
 /* Support native hash index */
 #define HA_CAN_HASH_KEYS        (1ULL << 58)
-#define HA_LAST_TABLE_FLAG HA_CAN_HASH_KEYS
+/*
+  Rowid's are not comparable. This is set if the rowid is unique to the
+  current open handler, like it is with federated where the rowid is a
+  pointer to a local result set buffer. The effect of having this set is
+  that the optimizer will not consirer the following optimizations for
+  the table:
+  ror scans or filtering
+*/
+#define HA_NON_COMPARABLE_ROWID (1ULL << 59)
+
+#define HA_LAST_TABLE_FLAG HA_NON_COMPARABLE_ROWID
 
 /* bits in index_flags(index_number) for what you can do with index */
 #define HA_READ_NEXT            1       /* TODO really use this flag */