Make pending assignments safe to race, settle them on a tick, and say only what was checked

Review of #150: a withdrawal could land between the look and the act, a
failed ask read as a build in flight, a request kept the wrong asker, a
build being registered read as not built, build "true" could ask a build
nothing waited on, and a status read changed state. Claim a row under the
machine's hold before making it, keep a request only once asked, settle on
the controller's own tick, raise an assignment not made as a condition
until it is answered, and tie each row to its machine.

cmd/mesh-controller/queue.go

@@ -406,6 +406,8 @@ func rebuildCommand(ctx context.Context, args []string) error {
 	if err != nil {
 		return err
 	}
+	recordAsked(ctx, inventory.BuildRequest{ID: id, Repository: source.Repository, Seat: source.Seat, Path: path,
+		Ref: ref, For: "rebuild"})
 	fmt.Printf("rebuild asked as %s\n", id)
 	return nil
 }
@@ -490,7 +492,11 @@ func replayCommand(ctx context.Context, args []string) error {
 	if err := replayRefusal(b, module, history, *register, *older, outstanding); err != nil {
 		return err
 	}
-	id, err := buildOneAsked(ctx, source, b.Path, b.Commit, 0, !*register)
+	asker := ""
+	if *register {
+		asker = "replay"
+	}
+	id, err := buildOneAsked(ctx, source, b.Path, b.Commit, 0, !*register, asker)
 	if err != nil {
 		return err
 	}