Merge branch 'jason/NNS1-3069-2' into 'master'

refactor(nns): NNS1-3069 Let neuron data model have dissolve_state_and_age instead of 2 fields

Motivation: like some of the other fields (id, controller, subaccount), we want our data model to match our mental model of the neuron (id/subaccount/controller always exists). The combined field dissolve_state_and_age is closer to our mental model since the 2 fields are closely related. 

See merge request dfinity-lab/public/ic!19345