Make lbevreactor remove connection from new connections list on discard

WHen connection is destroyed it is not removed from `self._new_conns`,
which is why the following error happens:
```
  File "/123/cassandra/io/libevreactor.py", line 197, in _loop_will_run
    conn._read_watcher.start()
AttributeError: 'NoneType' object has no attribute 'start'
```

Author: dkropachev

cassandra/io/libevreactor.py

@@ -165,6 +165,10 @@ def connection_created(self, conn):
 
     def connection_destroyed(self, conn):
         with self._conn_set_lock:
+            new_conns = self._new_conns.copy()
+            new_conns.discard(conn)
+            self._new_conns = new_conns
+
             new_live_conns = self._live_conns.copy()
             new_live_conns.discard(conn)
             self._live_conns = new_live_conns