The access tokens are cached for one minute, and up to 1000 different
users' access tokens will be cached at any one time.
The access token's cache per username remains idle before expiring.
Its expiration time depends response lifetime, and
expiration time is updated after looking up the cache.
Currently response lifetime is 1 minute.
Since I want not to access Active Directory frequently,
I make this response lifetime to be input-able for users.