IMPALA-5182: Explicitly close connection to impalad on error from shell

When using connections secured with SSL, a connection close comprises
of a bi-directional SSL_shutdown(). The second part of the
bi-directional shutdown requires that the client also close the socket
explicitly, and the server blocks till it gets the close
notification from the client.

This patch ensures that the above happens. Without this fix, the
impala-shell was found to hang over connections secured with SSL
when an error was encountered.

Change-Id: I814df93bbcd457ad3f96b4c1ef5d8b0ddd6d141f
Reviewed-on: http://gerrit.cloudera.org:8080/6587
Reviewed-by: Sailesh Mukil <sailesh@cloudera.com>
Tested-by: Impala Public Jenkins

shell/impala_shell.py

@@ -705,6 +705,8 @@ def _connect(self):
         self.prompt = self.DISCONNECTED_PROMPT
     except Exception, e:
       print_to_stderr("Error connecting: %s, %s" % (type(e).__name__, e))
+      # A secure connection may still be open. So we explicitly close it.
+      self.imp_client.close_connection()
       # If a connection to another impalad failed while already connected
       # reset the prompt to disconnected.
       self.server_version = self.UNKNOWN_SERVER_VERSION