This PR extracts two responsibilities from the `Database` class:
1. Managing locks for prefixes during an installation
2. Marking installation failures
and pushes them into their own class (`SpecLocker` and `FailureMarker`). These responsibilities are also pushed up into the `Store`, leaving to `Database` only the duty to manage `index.json` files.
`SpecLocker` classes no longer share a global list of locks, but locks are per instance. Their identifier is simply `(dag hash, package name)`, and not the spec prefix path, to avoid circular dependencies across Store / Database / Spec.
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