WIP: GH-20: Create server cache

[etandon-coatue]

No description provided.

[yilinwei]

Hey! Thanks for opening this PR; Meetup has been down frequently so it would be nice to have some cache for it 😄.

There are a couple of points to consider:

1. It would probably be best if we extracted the logic into a separate class and passed it into the apply method. This would mean that it's easier to test the cache behavior as an independent unit
2. In general we avoid mutable state unless there's a good reason to have it (i.e. bottleneck for performance). We can use a Ref with an immutable map to get a primitive concurrent cache.
3. In general, Map doesn't provide any guarantees about it's behaviour under concurrent conditions; looking at the implementation of Scala's Map, I think it would be fine, but it's not great to rely on implementation details.

I think sketching out how the solution might be used first before implementing it is a good idea. Here's a starting point.

trait Cache[F[_], K, V] {
  def fetch(key: K, retrieve: K => F[V]): F[V]
}

object Cache {
  def apply[F[_]: Sync, K, V](evictionTime: Duration): Cache[F, K, V] = { 
    ???
  }
}

Thanks for joining the gitter chat and let me know if you encounter any issues.