Add a pgbouncer.pool_mode GUC to disable certain Postgres functionality

[JelteF]

Certain Postgres features are not supported when using PgBouncer in transaction pooling mode. However, because PgBouncer does not inspect queries it is unable to block usage of those features. It would be great to add a pgbouncer.pool_mode GUC that when set to 'transaction' would cause Postgres to throw errors for the following things (taken from https://www.pgbouncer.org/features.html#sql-feature-map-for-pooling-modes):

• Query level prepared statements, i.e. DEALLOCATE/PREPARE
• Session level advisory locks, i.e. pg_advisory_lock/pg_advisory_unlock
• Session level SET/RESET commands for GUCs that are not GUC_REPORT. (probably means we want to implement #12 first)
• Listening for changes on channels using LISTEN (but still allow NOTIFY to report a change).
• WITH HOLD CURSOR
• PRESERVE ROWS/DELETE ROWS temp tables