Add a cache for remote IdP tokens to prevent un-necessary round-trips calls to the IdP

Related to #60 (closed). Closing as duplicate of oak-tree/medical-imaging/sonador#68.

Cache token in a session to prevent un-necessary round-trips (cache period should be the same as the expiry time of the token). When caching token credentials in a session, the token value should be hashed using one-way password hash so that Sonador isn't storing sensitive values.