Avoid blocking IO threads on CacheInputStream

As part of ISPN-10376, the REST server is serving keys using Netty's ChunkedWriteHandler that will convert a InputStream into chunks and will write it to the client.

As the source of the InputStream is a CacheStream, that can block, it's desirable to do the retrieval of the cache data in a separate thread, using the suspend and resume capabilities of the ChunkedWriteHandler when data is available

https://netty.io/4.0/api/io/netty/handler/stream/ChunkedWriteHandler.html#resumeTransfer--