Make ClusterStateCache use less memory when not storing attribute values.

ClusterStateCache has a "no storing values" mode, but in that mode it still uses
24 bytes per attribute to store just the data size.  For a typical device with a
few hundred attributes, this adds up to multiple KB of RAM; across many devices
it really starts to add up.

Introduce a compile-time switch that lets us optimize the storage when we know
we're not storing the data.

This still keeps the old "enable storing data at compile time but turn it off at
runtime" API, both for API compat and because it might reduce codesize for some
consumers who care about that more than RAM.

Author: bzbarsky-apple