Make data node command execution interruptible

The function to execute remote commands on data nodes used a blocking
libpq API that doesn't integrate with PostgreSQL interrupt handling,
making it impossible for a user or statement timeout to cancel a
remote command.

Refactor the remote command execution function to use a non-blocking
API and integrate with PostgreSQL signal handling via WaitEventSets.

Partial fix for #4958.

CHANGELOG.md

@@ -10,6 +10,7 @@ accidentally triggering the load of a previous DB version.**
 * #5241 Allow RETURNING clause when inserting into compressed chunks
 * #5245 Mange life-cycle of connections via memory contexts
 * #5246 Make connection establishment interruptible
+* #5253 Make data node command execution interruptible
 
 **Bugfixes**
 * #4804 Skip bucketing when start or end of refresh job is null

tsl/src/data_node.c

@@ -257,7 +257,6 @@ data_node_get_connection(const char *const data_node, RemoteTxnPrepStmtOption co
 	const ForeignServer *server;
 	TSConnectionId id;
 
-	Assert(data_node != NULL);
 	server = data_node_get_foreign_server(data_node, ACL_NO_CHECK, false, false);
 	id = remote_connection_id(server->serverid, GetUserId());