Add setting connected_event flag in libevreactor

Before, the connected_event flag was set in every implementation
of Connection but this utilizing libev. Other reactors have the
same `self.error_all_requests(ConnectionShutdown(...))` logic, but
they have `self.connected_event.set()` after that, so it was probably
an oversight (copy-paste mistake?) that it was missing from this
reactor. That was causing the driver to sometimes hang for >3 minutes
when shutting down.

This commit adds setting the `connected_event` flag in `close()` in
`LibevConnection`.

Author: sylwiaszunejko

cassandra/io/libevreactor.py

@@ -294,6 +294,7 @@ def close(self):
         if not self.is_defunct:
             self.error_all_requests(
                 ConnectionShutdown("Connection to %s was closed" % self.endpoint))
+            self.connected_event.set()
 
     def handle_write(self, watcher, revents, errno=None):
         if revents & libev.EV_ERROR: