Expose Buf instead of ChannelBuffer in storehaus-redis

[BenFradet]

Since Finagle is moving away from exposing netty's ChannelBuffer in their API in favor of twitter util's Buf, I think we should too.

From the recent commits, I think the replacement should be complete by Finagle's next release.

[johnynek]

Yeah, I think this is a good call. (PS: lesson to be learned here, it is a good idea to hide all your dependencies on your public API in general. We didn't do that in a few places in scalding, I really regret it).