Make BuildReceptionIdentity public, and make backup restore function return a reception identity object

Initial params refactor. This doesn't fix anything but gets it into a state where one can make progress

This fixes a few issues:
1. When logging in, try to load the store then init if it doesn't exist.
2. Fix receptionIdentity return on LoginLegacy returning Transmission, not Reception, elements
3. Call NewStateLegacy inside the LoginLegacy object, which would otherwise break compatibility with existing mobile client stores.