ALTER/DROP SERVER requires FLUSH TABLES

ALTER/DROP SERVER used to walk the list of tables and flush affected tables automatically. This was broken since 2011, didn't flush partitioned tables. And was completely removed in MDEV-37815.

Needs to be reimplemented, for example by versioning server_cache and have engines reload server definition when the version changes. Or they can simply load the server definition for every query, Federated isn't exactly a high-performance engine and one hash lookup per query won't make any difference.