v0.13.0

Implements spec v0.43, small fixes from the reference's v0.42 notes.

Changes to check before upgrading:

• An operator key authorises only within its period: grants it signed are refused
  before its from as well as after its until (§C.4). A key configured with
  -operator-key and no history entry is unaffected.
• GET / answers "spec": "0.43".

The other v0.43 changes state what the reference already did: grants in sealed namespaces
are sealed under the epoch of the first entry recording them, repeated query parameters are
400, and a bundle line whose writing namespace is unknown carries neither written nor
grant.

Full Changelog: v0.12.0...v0.13.0