v0.12.0

Implements spec v0.42, which settles the reference's v0.41 notes on signatures.

Changes to check before upgrading:

• Unknown query parameters are 400 bad_input on every core endpoint, before
  authentication, as are repeated parameters and flag values other than 1
  (dry-run=true used to be ignored and the write made).
• The gestures listing pages with ?after=, as §7.4 says; it took ?since=. The Go
  client and the playground follow.
• Author signatures are checked at §6.2 step 2.3, after rate limits, the retry lookup
  and settling the verb: an idempotent retry is answered as first recorded whatever
  signature it carries, even after required was turned on; 403 and 429 come before
  422; a dry run reports 422 signature per item; a write without a usable precondition
  gets 428/400. A batch whose config change is stale fails with that change's 412.
• GET /ns/{ns}/grants/{gid} is offered in sealed namespaces (sealed, pl: { ns, grant },
  stored once) and end-to-end ones (clear, Cache-Control: private), and is 410 once the
  namespace is purged; not_offered is gone. client.Grant opens sealed answers.
• An operator key past its until authorises nothing; it used to be accepted after it.
• GET / answers "spec": "0.42".

Bundles and archives:

• A grant line's ns is the namespace whose entry first recorded the grant; key is
  optional, looked up in the namespace document at that entry, then the operator key history
  at the first naming revision's created, and left out otherwise.
• History lines for revisions written elsewhere (a branch's read-through, a remote branch's
  base) carry written, used in the signing input, so branch bundles verify.
• Sealed and end-to-end exports carry grant lines (ExportOptions.Bearer/Identity for
  sealed grants); a grant the exporter can't fetch is left off the lines that would name it.
• Pruning archives (§8.6) carry authors and grant lines; restore skips them.

Performance:

• Heads pages on Postgres take a few statements, not three per resource. A page now
  reads its resources, their heads at at (one LATERAL join on head_history) and those
  revisions in one statement each per namespace level, and resolves from memory.

Full Changelog: v0.11.1...v0.12.0