Move identities core code out of state package

[benhoyt]

Per discussion with Gustavo, we'd like to move the state.*Identities functions out of the core state package and into their own package (overlord/identitiesstate or overlord/identities) to avoid cluttering the core state package. They'll still be stored in the state database, but via state.Get and state.Set with an "identities" key.

Note that there will be a data migration associated with this, as currently the "identities" dict lives at the top level, whereas after this it'll live under "data". This means we'll need some legacy code pull it out on startup -- either as part of the state package or in a separate migration step.