Do not overwrite members that are already in cache during chunking.

discord/state.py

@@ -379,6 +379,7 @@ class ConnectionState:
         members = data.get('members', [])
         for member in members:
             m = self._make_member(server, member)
+            if m.id not in server._members:
                 server._add_member(m)
 
         # if the owner is offline, server.owner is potentially None