Add support for modern store interface

[nfriedly]

We need to update express-slow-down to support the v2 store interface that was introduced in express-rate-limit v6.

This could be done by either directly adding support, or by re-working express-slow-down to build on top of express-rate-limit, thereby inheriting it's store support.

Relates to #40

[nfriedly]

This is a bit more urgent now that at least one store has dropped the v1 interface entirely.