server_reset_query in session pool mode should be discard all

The pgbouncer docs say when using session pool mode, the reset query should be DISCARD_ALL.

http://pgbouncer.projects.pgfoundry.org/doc/faq.html#_what_should_my_server_reset_query_be.

Also adds support for PGBOUNCER_SERVER_RESET_QUERY env var.