Initial IDistributedCache abstraction and implementations.

This includes:
- The IDistributedCache abstraction
- An implementation using MemoryCache
- An implementation using Redis.

This does not include
- Any serialization helpers
- Any concurrency controls (optimistic, locking, etc.).

Notes:
- Resis does not have any notion of sliding expiration, only absolute expiration. It does allow a single entry to contain multiple key-value pairs of data. We use this to store the absolute and sliding expiration values with the data and retrieve them later when we want to re-calculate the expiration.
- Some redis operations are best implemented via LUA script sent to the server. I've done the critical ones that way so far, but there are a few more (like refresh) that could be optimized via script.
Here's a wild solution:
MemoryCacheOptions can implement `IOptionsAccessor<out MemoryCacheOptions>` and just return `this`. This makes the constructor DI friendly, while still letting you do the following:
```
new MemoryCache(new MemoryCacheOptions()
{
    ListenForMemoryPressure = false,
});
```