MDEV-29640  FederatedX does not properly handle pushdown in case of difference in local and remote table names

FederatedX table may refer to a table with a different name on the
remote server:
  test> CREATE TABLE t2 (...) ENGINE="FEDERATEDX"
  CONNECTION="mysql://user:pass@192.168.1.111:9308/federatedx/t1";
  test> select * from t2 where ...;
This could cause an issue with federated_pushdown=1, because FederatedX
pushes the query (or derived table's) text to the remote server. The remote
server will try to read from table t2 (while it should read from t1).

Solution: do not allow pushing down queries with tables that have different
db_name.table name on the local and remote server.

This patch also fixes:
MDEV-29863 Server crashes in federatedx_txn::acquire after select from the
FederatedX table with partitions

Solution: disallow pushdown when partitioned FederatedX tables are used.