[FLOC-3847] Prototype a dataset agent for remote filesystems like NFS

[itamarst]

This is a prototype; it will be thrown away and reimplemented using test-driven development.

Goals:

• Gather feedback from potential third parties who will wish to write plugins about interfaces and design goals using more concrete code as basis for discussion.
• Identify mismatches with the current model and implementation.
• Get general sense of how we can improve existing code to reuse it.

Work remaining:

• Actually hook up to flocker-dataset-agent and plugin mechanism.
• Get this to run at least once, if that is useful.