Enforce search_path=pg_catalog for non-replication connections (#2496)

There is a known [vector of attact](https://pganalyze.com/blog/5mins-postgres-security-patch-releases-pgspot-pghostile) by creating functions and/or operators in a public scheme with the same name and signature as corresponding objects in `pg_catalog`.

Since Patroni is heavily relying on superuser connections we want to mitigate it by enforcing `search_path=pg_catalog` for all connections created by Patroni (except replication connections). It is achieved by introducing a new function, that wraps psycopg.connect() and appends ` -c search_path=pg_catalog` to `options` parameter.

In addition to that, we set connection.autocommit to True before returning it.

tests/__init__.py

@@ -177,7 +177,7 @@ class PostgresInit(unittest.TestCase):
                    'force_parallel_mode': '1', 'constraint_exclusion': '',
                    'max_stack_depth': 'Z', 'vacuum_cost_limit': -1, 'vacuum_cost_delay': 200}
 
-    @patch('patroni.psycopg.connect', psycopg_connect)
+    @patch('patroni.psycopg._connect', psycopg_connect)
     @patch('patroni.postgresql.CallbackExecutor', Mock())
     @patch.object(ConfigHandler, 'write_postgresql_conf', Mock())
     @patch.object(ConfigHandler, 'replace_pg_hba', Mock())