Exception with pgBouncer

## Setup
EMQX: v4.0.0
Environment: k8s (installed with Helm)

## Problem
`emqx_auth_pgsql` crashes with postgres server that is accessed via pgbouncer (in Transaction Mode). It used to work fine when using it directly with postgres server.

The crash error is as follows :
```
(emqx@emqx-0.emqx-headless.nh-production.svc.cluster.local)1> 2020-07-31 06:36:32.039 [error] <<"8_69_esp32_d9ab10">>@10.244.6.56:56432 [Postgres] query '"auth_query"' failed: { │
│ invalid_sql_statement_name, │
│ <<"prepared statement \"auth_query\" does not exist">>, │
│ [{file,<<"prepare.c">>}, │
│ {line,<<"512">>}, │
│ {routine, │
│ <<"FetchPreparedStatement">>}, │
│ {severity,<<"ERROR">>}]}
```

Should changing the mode of pgBouncer to Session or something else help with this issue?