postgres_fdw: Fix handling of pending asynchronous requests.

A pending asynchronous request is handled by process_pending_request(),
which previously not only processed an in-progress remote query but
performed ExecForeignScan() to produce a tuple to return to the local
server asynchronously from the result of the remote query.  But that led
to a server crash when executing a query or led to an "InstrStartNode
called twice in a row" or "InstrEndLoop called on running node" failure
when doing EXPLAIN ANALYZE of it, in cases where the plan tree for it
contained multiple async-capable nodes accessing the same
initplan/subplan that contained multiple async-capable nodes scanning
the same foreign tables as for the parent async-capable nodes, as
reported by Andrey Lepikhov.  The reason is that the second step in
process_pending_request() invoked when executing the initplan/subplan
for one of the parent async-capable nodes caused recursive execution of
the initplan/subplan for another of the parent async-capable nodes.

To fix, split process_pending_request() into the two steps and postpone
the second step until ForeignAsyncConfigureWait() is called for each of
the pending asynchronous requests.  Also, in ExecAppendAsyncEventWait()
we assumed that FDWs would register at least one wait event in a
WaitEventSet created there when they were called from
ForeignAsyncConfigureWait() in that function, but allow FDWs to register
zero wait events in the WaitEventSet; modify ExecAppendAsyncEventWait()
to just return in that case.

Oversight in commit 27e1f1456.  Back-patch to v14 where that commit went
in.

Andrey Lepikhov and Etsuro Fujita

Discussion: https://postgr.es/m/fe5eaa19-1704-e4a4-76ee-3b9d37ade399@postgrespro.ru

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -10300,6 +10300,59 @@ SELECT * FROM async_pt t1, async_p2 t2 WHERE t1.a = t2.a AND t1.b === 505;
  2505 | 505 | 0505 | 2505 | 505 | 0505
 (1 row)
 
+CREATE TABLE local_tbl (a int, b int, c text);
+INSERT INTO local_tbl VALUES (1505, 505, 'foo');
+ANALYZE local_tbl;
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM local_tbl t1 LEFT JOIN (SELECT *, (SELECT count(*) FROM async_pt WHERE a < 3000) FROM async_pt WHERE a < 3000) t2 ON t1.a = t2.a;
+                                       QUERY PLAN                                       
+----------------------------------------------------------------------------------------
+ Nested Loop Left Join
+   Output: t1.a, t1.b, t1.c, async_pt.a, async_pt.b, async_pt.c, ($0)
+   Join Filter: (t1.a = async_pt.a)
+   InitPlan 1 (returns $0)
+     ->  Aggregate
+           Output: count(*)
+           ->  Append
+                 ->  Async Foreign Scan on public.async_p1 async_pt_4
+                       Remote SQL: SELECT NULL FROM public.base_tbl1 WHERE ((a < 3000))
+                 ->  Async Foreign Scan on public.async_p2 async_pt_5
+                       Remote SQL: SELECT NULL FROM public.base_tbl2 WHERE ((a < 3000))
+   ->  Seq Scan on public.local_tbl t1
+         Output: t1.a, t1.b, t1.c
+   ->  Append
+         ->  Async Foreign Scan on public.async_p1 async_pt_1
+               Output: async_pt_1.a, async_pt_1.b, async_pt_1.c, $0
+               Remote SQL: SELECT a, b, c FROM public.base_tbl1 WHERE ((a < 3000))
+         ->  Async Foreign Scan on public.async_p2 async_pt_2
+               Output: async_pt_2.a, async_pt_2.b, async_pt_2.c, $0
+               Remote SQL: SELECT a, b, c FROM public.base_tbl2 WHERE ((a < 3000))
+(20 rows)
+
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+SELECT * FROM local_tbl t1 LEFT JOIN (SELECT *, (SELECT count(*) FROM async_pt WHERE a < 3000) FROM async_pt WHERE a < 3000) t2 ON t1.a = t2.a;
+                                       QUERY PLAN                                        
+-----------------------------------------------------------------------------------------
+ Nested Loop Left Join (actual rows=1 loops=1)
+   Join Filter: (t1.a = async_pt.a)
+   Rows Removed by Join Filter: 399
+   InitPlan 1 (returns $0)
+     ->  Aggregate (actual rows=1 loops=1)
+           ->  Append (actual rows=400 loops=1)
+                 ->  Async Foreign Scan on async_p1 async_pt_4 (actual rows=200 loops=1)
+                 ->  Async Foreign Scan on async_p2 async_pt_5 (actual rows=200 loops=1)
+   ->  Seq Scan on local_tbl t1 (actual rows=1 loops=1)
+   ->  Append (actual rows=400 loops=1)
+         ->  Async Foreign Scan on async_p1 async_pt_1 (actual rows=200 loops=1)
+         ->  Async Foreign Scan on async_p2 async_pt_2 (actual rows=200 loops=1)
+(12 rows)
+
+SELECT * FROM local_tbl t1 LEFT JOIN (SELECT *, (SELECT count(*) FROM async_pt WHERE a < 3000) FROM async_pt WHERE a < 3000) t2 ON t1.a = t2.a;
+  a   |  b  |  c  |  a   |  b  |  c   | count 
+------+-----+-----+------+-----+------+-------
+ 1505 | 505 | foo | 1505 | 505 | 0505 |   400
+(1 row)
+
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT * FROM async_pt t1 WHERE t1.b === 505 LIMIT 1;
                            QUERY PLAN                           
@@ -10342,8 +10395,6 @@ SELECT * FROM async_pt t1 WHERE t1.b === 505 LIMIT 1;
 (1 row)
 
 -- Check with foreign modify
-CREATE TABLE local_tbl (a int, b int, c text);
-INSERT INTO local_tbl VALUES (1505, 505, 'foo');
 CREATE TABLE base_tbl3 (a int, b int, c text);
 CREATE FOREIGN TABLE remote_tbl (a int, b int, c text)
   SERVER loopback OPTIONS (table_name 'base_tbl3');