Removes a lot of unused (in ABS) functionality, refactors to ES6
style class, and re-enables this custom implementation with check
period and ttl of 1 day, and 1000 max entries.
The class now only implments the required (as per express-session docs)
methods and removes optional methods, except touch() which allows the
TTL of an entry to be refreshed without affecting its LRU recency.
There is no longer a way to stop the prune timer, but I don't belive
the function was ever being called beforehand. The session store's
lifetime is the same as the application's, and since it is unref()'d
should not cause any shutdown issues.
