"pg_ctl stop" stops working after a backend crash

Hi hackers,

I noticed that sometimes, when I'm running the regression tests and a
backend crashes, the postmaster can get stuck in some weird state
where it doesn't terminate and doesn't respond to `pg_ctl stop`
anymore. I can semi-reliably reproduce this on 18.3 using a simple
script below.

```
set -e
pg_ctl start -w

psql -X -d postgres -c "SELECT pg_sleep(60)" &>/dev/null &
sleep 0.3
VICTIM=$(psql -X -d postgres -tAc \
"SELECT pid FROM pg_stat_activity WHERE query LIKE '%pg_sleep%' LIMIT 1")

kill -9 "$VICTIM" # trigger crash recovery
sleep 0.01 # let postmaster start reinitializing
timeout 8 pg_ctl stop -m fast &
STOP_PID=$!

sleep 6
if kill -0 "$STOP_PID" 2>/dev/null; then
echo "pg_ctl stop froze. Active processes:"
pgrep -al postgres
exit 1
fi
echo "Successful shutdown"
```

A typical output of this is:

598814 postgres
598864 postgres: io worker 0
598865 postgres: io worker 1
598866 postgres: io worker 2
598868 postgres: checkpointer

These processes just stay there indefinitely, and the shutdown
finishes if I do `pkill -USR2 postgres`.

The not working pg_ctl looks like a bug, so I though I should ask for
your comment on this.

Best regards
Alexander Kuzmenkov
TigerData