BUG #14367: Pressing execute 20-30 times really fast causes loop of error messages

The following bug has been logged on the website:

Bug reference: 14367
Logged by: Jan Kort
Email address: jan(dot)kort(at)genetics(dot)nl
PostgreSQL version: 9.6.0
Operating system: Windows 10
Description:

-- Execute (F5) the first 3 lines of this script once
DROP TABLE IF EXISTS TEST_JAN;
CREATE TABLE TEST_JAN (F1 TEXT);
INSERT INTO TEST_JAN (F1) VALUES ('Test');
-- Execute (F5) the next line 20-30 times really fast, causes loop saying:
connection.Cursor could not be found for the async
INSERT INTO TEST_JAN (SELECT * FROM TEST_JAN);
-- Execute (F5) the next line shows 4096 records inserted
SELECT COUNT(*) FROM TEST_JAN;
-- Other issues:
-- 1. executing trivial queries take 400ms or more, used to be 15ms
-- 2. I was trying to reproduce another issue with this example, but I
can't.
-- The other issues was that sometimes the progres of the "Query Tool"
keeps on going
-- eventhough the query is quick enough. When I restart the querytool,
the exact same
-- query executes fine (400ms). This happens maybe 1 in 20 times, but not
with simple
-- examples, so I dont know..