Consider not claiming intermediate result connections exclusively

When there are multiple levels of recursive planning (hence broadcasting intermediate results), Citus claims one connection per level exclusively via `ClaimConnectionExclusively()` in `RemoteFileDestReceiverStartup()`. This means that, if there are X levels of planning, Citus would use X connections per node.

Instead, we could optimize the overall execution to use 1 connection per node. The following is a quote from the chat with @marcocitus

```

I think RemoteFileDestReceiverShutdown only happens once the executor is done with the outer query
​ and the entire inner query execution happens within the execution of the outer query
​ technically, I think we might be able to defer opening connections from RemoteFileDestReceiverStartup to RemoteFileDestReceiverReceive
​ so we rather wait with broadcasting the outer query until we have results from the outer query
​ since the inner query finishes execution before the outer query starts, that should work
​ that could be a useful optimisation for nested recursive plans

```

An example query for nested ctes (`random()` is to prevent CTE inlining ):
```SQL
WITH cte_1 AS (WITH cte_2 AS (SELECT user_id, random() FROM users_table) SELECT user_id,random() FROM users_table JOIN cte_2 USING (user_id))

SELECT count(*) FROM cte_1 JOIN users_table USING (user_id);

```