session存储在mysql中，当cookie过期了，怎么删除msyql中的session？
You can store the session content in external stores (Redis, MongoDB or other DBs) by passing options.store with three methods (these need to be async functions):

get(key, maxAge, { rolling }): get session object by key
set(key, sess, maxAge, { rolling, changed }): set session object for key, with a maxAge (in ms)
destroy(key): destroy session for key

这里不能获取失效的key