Servers for a scale-out clustering setup can now be added and deleted
with `add_server()` and `delete_server()`, providing a convenience API
for server management.
While similar functionality can be achieved using the standard
PostgreSQL `CREATE SERVER` and `CREATE USER MAPPING` commands, this
new API makes it easier to add clustering servers and user mappings
consistent with the needs of TimescaleDBs particular clustering setup.
The API currently works with the `postgres_fdw` foreign data
wrapper. It will be updated to use our own foreign data wrapper once
it is available.
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