connection is None in _remove_idle_unusable_session after session.close()

[liuxiaocs7]

I have implement session pool feature to 2.6 refer to the session pool implementation in the master branch (#246), and i find the sessionpool may have the same issue.

after session.release(), session.connection is None, so session._connection.close() will raise an error

nebula-python/nebula3/gclient/net/SessionPool.py

Lines 458 to 461 in f028dfc

	if idle_time > self._configs.idle_time:
	session.release()
	session._connection.close()
	self._idle_sessions.remove(session)

nebula-python/nebula3/gclient/net/Session.py

Lines 228 to 237 in f028dfc

	def release(self):
	"""release the connection to pool, and the session couldn't been use again
	:return:
	"""
	if self._connection is None:
	return
	self._connection.signout(self._session_id)
	self._connection.is_used = False
	self._connection = None