Act under a lease, keep accounts by order, one writer at composition (hq to-be 45 Phase 2)

Two controllers could both act (issue 204), a reconcile's report could
overtake the apply after it and the digest decided (issue 267), and a grant
could make a second writer of a machine's report.

- The lease (internal/lease, ADR 0229): mesh-controller_lease key `holder`,
  15 s age, renewed every 5 s by compare-and-set; the epoch is the revision
  it was taken at. The gate is the clock (stops 3 s before expiry); a refused
  renewal is a loss and the process exits; a holder that stops gives it back.
  serve takes it before asserting the bus. Epochs kept in the store
  (migration 0068 controller_epoch) as a floor: a bucket raised from nothing
  is compacted past it. Unleased (no epoch, S12 urgent) only when nobody
  holds it and the bus will not let it be written. A shell command acts
  under the holder's epoch, or its own lease when none.
- Declarations carry `epoch` inside the signed envelope, only to a machine
  whose latest account carried a report_sequence (mesh-host #35); would-send
  is composed with the epoch last sent. Allot and the send both pass the gate.
- Reports: contract in internal/link/order.go (epoch, sequence,
  report_sequence, older_than, refused_older). Accounts kept by epoch, then
  sequence, then report sequence; older refused, counted; unordered reports
  keep the digest rule. Plans by compare-and-set on a revision, with epoch.
  Conditions and calls carry the epoch and are not written off the lease.
- S12 and S13 (naming the writer by epoch) watched, D5 run; reset of the
  bucket said. Writers table compiled in and enforced in PermissionsFor; the
  controller no longer publishes mesh.control.>. A contract per consumed
  kind, and the empty-on-error lint over the repository.
- mesh-host pinned to its main with the epoch in the validator (D1 validates
  the envelope as sent).

Needs mesh-host's genesis lock with the lease grant (mesh-host PR) for
TestTheInstallersFirstUserListIsWhatTheControllerWouldCompose.

cmd/mesh-controller/held_back_test.go

@@ -128,7 +128,7 @@ func (r *recordedDelivery) grant(context.Context, []readyNode) error { return ni
 
 func (r *recordedDelivery) declare(ctx context.Context, s readyNode, body []byte) (string, error) {
 	r.declared = append(r.declared, s.node)
-	return recordSent(ctx, r.inv, s.node, body, s.declared.Builds)
+	return recordSent(ctx, r.inv, s.node, body, s.declared.Builds, s.declared.Epoch)
 }
 
 // aResolver is a module built from a repository, at a commit, with something on the machine that