Query cancellation problem

pgx/v4 does not work as expected with pgbouncer due to query cancellation (issue #679). In order to replicate:

1. docker-compose up
2. echo "show pools;" | psql -h localhost -p 6432 -U postgres pgbouncer

Output:

database	user	cl_active	cl_waiting	sv_active	sv_idle	sv_used	sv_tested	sv_login	maxwait	maxwait_us	pool_mode
pgbouncer	pgbouncer	1	0	0	0	0	0	0	0	0	statement
postgres	postgres	2	97	0	0	0	0	1	0	748640	transaction

The test runs 100 workers each sending queries that are always canceled. Pgbouncer's pool of client connections is limited to 100, service's pool is limited to 10. One would expect the total number of connections equal 10, but here we have 100.