Complete session state cleanup for DISCARD ALL

[murex971]

PgDog intercepts DISCARD ALL and responds without forwarding it to PostgreSQL. Prepared statements are cleared, but other session state documented by PostgreSQL is still retained.
DISCARD ALL should also:

• remove all LISTEN subscriptions (UNLISTEN *);
• release all advisory locks held by the client;
• reset session parameters to the values supplied in the startup message.

This should make a PgDog session behave like a PostgreSQL session after DISCARD ALL.
Reference: https://www.postgresql.org/docs/current/sql-discard.html