Daemon state stores in SQLite (state.db) #624

The problem

The daemon keeps its state in JSON files: state.json (devices, leases, lease requests), tokens.json, workers.json and the gateway's drain list. Every registry change rewrites the whole state.json. Stores that are read, changed and written back can lose a concurrent change (#616 was one).

What you would like to happen

Move these stores into one SQLite file of their own (state.db), separate from the event history (#623 moves that into history.db). A damaged state.db refuses to start, as a corrupt registry does today.

Anything else

• Raised in the spec session of Operational metrics kept over hours and shown in the console #623, which keeps this out of its scope.
• instance.json, adb-server.json, config.json, admin.token and daemon.log stay plain files (ADR 0001, ADR 0003).
• Open for the spec: migrating live leases on upgrade, the startup safety checks, downgrades, and the registry tests that run on a fake filesystem.

Written by an agent.