Cache chunked rather than whole byte[]

[benaadams]

Cached data should be stored in < 85k chunks rather than as whole byte[] so they live in GC generations rather than LoH and can be cleaned without requiring a full Gen2

[benaadams]

Fragmented stream example https://github.com/Microsoft/Microsoft.IO.RecyclableMemoryStream

[JunTaoLuo]

Sharding has been implemented in #57 for MemoryResponseCacheStore.

[benaadams]

@JunTaoLuo very nice, thank you!