Time-updatable keys
Hypothetical node-level updatable key. Might have a version number or a time. Derived from SignedSubspaceKey .
Nodes can store multiple versions of the key.
Clients can fetch either:
- All known versions of the TUK.
- The latest version of the TUK.
- A specific version number of the TUK.
- (Possibly) the TUKs between two times.
