Add option to publish metrics to a memory-mapped file

[mattrjacobs]

I'd like to explore how efficient metrics publishing could be is Hystrix exposed a way to publish metrics to a memory-mapped file. Then, another process could share that memory, and be dedicated to shoveling the bits onto the wire. This would allow any architecture to be used for this process (possibly Netty) without any changes required to the app producing Hystrix metrics