Currently there are several places in citus that map placements to
connections and that manage placement health. Centralize this
knowledge. Because of the centralized knowledge about which
connection has previously been used for which shard/placement, this
also provides the basis for relaxing restrictions around combining
various forms of DDL/DML.
Connections for a placement can now be acquired using
GetPlacementConnection(). If the connection is used for DML or DDL the
FOR_DDL/DML flags should be used respectively. If an individual
remote transaction fails (but the transaction on the master succeeds)
and FOR_DDL/DML have been specified, the placement is marked as
invalid, unless that'd mark all placements for a shard as invalid.
Andres Freund