Allow explicit `running` / `candidate` / `operational` DSes as a target for various operations

[jktjkt]

On an NMDA server, we're currently directing all writes to candidate and reads to operational. This has all sorts of nasty surprises, like a set not being visible via a get, etc. Let's at least make this explicit by allowing the data-accessing operations to target a given DS explicitly, or change this in the switch command, etc (and maybe do the right thing by not offering this behavior by default)