Choose synchronous nodes based on replication lag (#1786)

This commit makes it possible to configure the maximum lag (`maximum_lag_on_syncnode`) after which Patroni will "demote" the node from synchronous and replace it with another node.

The previous implementation always tried to stick to the same synchronous nodes (even if they are not optimal ones).

features/environment.py

@@ -194,6 +194,8 @@ class PatroniController(AbstractController):
         if custom_config is not None:
             self.recursive_update(config, custom_config)
 
+        self.recursive_update(config, {
+            'bootstrap': {'dcs': {'postgresql': {'parameters': {'wal_keep_segments': 100}}}}})
         if config['postgresql'].get('callbacks', {}).get('on_role_change'):
             config['postgresql']['callbacks']['on_role_change'] += ' ' + str(self.__PORT)