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/release_plan.go

@@ -344,7 +344,8 @@ var askABuild func(ctx context.Context, source buildSource, path, ref string) (s
 
 func init() {
 	askABuild = func(ctx context.Context, source buildSource, path, ref string) (string, error) {
-		return buildOneAsked(ctx, source, path, ref, 0, false)
+		// Kept by each asker with its own name once the ask is made (novox/hq issue 325).
+		return buildOneAsked(ctx, source, path, ref, 0, false, "")
 	}
 }
 
@@ -376,6 +377,8 @@ func askModule(ctx context.Context, p *inventory.Plan, name string, byName map[s
 		p.Note = fmt.Sprintf("%s could not be asked for: %v", name, err)
 		return
 	}
+	recordAsked(ctx, inventory.BuildRequest{ID: id, Repository: e.Source.Repository, Seat: e.Source.Seat,
+		Path: e.Source.Path, Ref: followedBranch(e.Source.Ref), Commit: p.Commit, For: "plan"})
 	state.State = "asked"
 	state.AskedAt = &now
 	state.Build = id