Add new config to ensure backup/restore connects to PG directly.

* In `pg_dump` 10.3+ and 9.5.12+, in
  it does a `SELECT pg_catalog.set_config('search_path', '', false)`
  which changes the state of the current connection. This is known
  to be problematic with Pgbouncer which reuses connections. As such,
  we'll always try to connect directly to PG directly during
  the backup/restore process.

config/discourse_defaults.conf

@@ -30,9 +30,16 @@ db_socket =
 # This is set to blank so it tries to use sockets first
 db_host =
 
+# host address for db server when taking a backup via `pg_dump`
+# Defaults to `db_host` if not configured
+db_backup_host =
+
 # port running db server, no need to set it
 db_port =
 
+# db server port to use when taking a backup via `pg_dump`
+db_backup_port = 5432
+
 # database name running discourse
 db_name = discourse