Saves the value to the specified cache key. Optionally, an expiry in seconds can be specified. If the value is omitted, an implementation-details dependant default expiration is assumed.
When implementing this interface, document what the default value for the expiry is.
Optional expiry: numberGenerated using TypeDoc
Returns the cached/saved value for the specified cache key. If there was never a set for the same cache key, undefined is returned.
If the value saved for the cache key is already expired, it is assumed to be not present, and undefined is returned.