Durable data

I'm not looking to actually merge this PR, just get some feedback on two possible approaches.

If there is agreement on one approach, then I will code it up, and convert this design document into user documentation.  

I'm inclined to add a new type of REST resource to model Durable local data.  I know new resources are always  controversial, so I've written this doc to compare the alternatives.  Comments welcome from anyone.  I'd particularly like comments from @dchen1107 @bgrant0607 @thockin @brendanburns @jbeda @lavalamp @smarterclayton.

Other requirements:
* No external orchestration should be required in order to set up the durable storage volume (e.g., creating/mounting host directories)
* It should be possible to update a pod (e.g., change a container's image version) without losing the durable data