Added LagAwareHealthCheck for MultiDBClient (#3737)

* Added LagAwareHealthcheck

* Added testing for LagAwareHealthCheck

* Fixed timeouts

* Added lag tollerance parameter

* Decreased messages_count due to increased timeouts

* Added docblocks

* Added missing type hints

* Fixed url

* Refactored tests, URL and cluster support

* Use primary node to send an API request

* Added comment about RE bug

* Moved None type to the beginning

* Added health_check_url property to Database class

Author: vladvildanov

redis/cluster.py

@@ -691,6 +691,7 @@ def __init__(
             self._event_dispatcher = EventDispatcher()
         else:
             self._event_dispatcher = event_dispatcher
+        self.startup_nodes = startup_nodes
         self.nodes_manager = NodesManager(
             startup_nodes=startup_nodes,
             from_url=from_url,