Mark nodes returning InvalidReplicaSetConfig in heartbeats as down

The sender of heartbeats requests doesn't mark the nodes that return InvalidReplicaSetConfig errors as down or up, the state of that node remains the previous state - Unknown or Down. We should mark the node as down while keeping sending heartbeats to it.
 

Original description:

Currently a removed node responds InvalidReplicaSetConfig to heartbeat requests. However that cannot be distinguished from real invalid replica set configs, so that disallow_adding_initialized_node2.js has to keep sending heartbeats even if the config is invalid. We could also give it a new error code if we think NodeNotFound is not descriptive.