Don't store db credentials in file

[mgrunbauer]

This PR introduces pgbouncer to setup connection pooling for Postgres.

It uses the ADAGUC_DB environment variable so that pgbouncer knows where to connect to, and adaguc-server can then connect to localhost. To make pgbouner work, the username and password (from ADAGUC_DB) get stored in the userlists.txt, which gets written by run_supervisord.sh.

Ideally, the username and password would not get stored in a file, but kept in an environment variable which pgbouncer could use to connect.

Alternatively, maybe it is possible to omit the userlist.txt and store user/pass directly in the pgbouncer.ini file.