CAS operation on configs

[jml]

From @jml on January 24, 2017 18:30

Would be good to have a compare-and-swap operation on configs to enable reliable tools for automatically updating them.

@tomwilkie I recall you mentioning this in the context of a particular use-case. Can you remember what it is?

Copied from original issue: weaveworks/service#1084

[jml]

From @tomwilkie on January 24, 2017 18:34

We were discussing dashboards / notebooks - we could use the config server to serve them, and if we do we should ensure its hard to accidentally overwrite a concurrent edit.

[gouthamve]

Closed in #649