Cause: shared federatedx_io cannot store table-specific data.

Fix: move current row reference `federatedx_io_mysql::current` to
ha_federatedx.

FederatedX connection (represented by federatedx_io) is stored into
federatedx_txn::txn_list of per-server connections (see
federatedx_txn::acquire()). federatedx_txn object is stored into THD
(see ha_federatedx::external_lock()). When multiple handlers acquire
FederatedX connection they get single federatedx_io instance. Multiple
handlers do their operation via federatedx_io_mysql::mark_position()
and federatedx_io_mysql::fetch_row() in arbitrarty manner. They access
the same federatedx_io_mysql instance and same MYSQL_ROWS *current
pointer, so one handler disrupts the work of the other.

Related to "MDEV-14551 Can't find record in table on multi-table update
with ORDER BY".