Keep rollouts and the merges they answer in the rollouts bucket (issue 1092)

Once chain_moved records the kind, a plan is a key of the rollouts bucket under its id and the open batch is the same key in state assembling. A save refuses a copy another save moved since (the plan's own Revision, kept in its value, counts saves) and keeps the merges the record holds, so a copy read before a merge moved cannot lay it back. The merges a batch or walk answers are the record's held field, with the columns the table had; a merge no plan answers stays, flagged, in the record that held it last, or in the record kept for those. A merge moved to a plan is written to the receiving record first and then taken from the giving one; one found in both is read from the one it entered last and repaired at the next cut. Plans and merges are read from a view kept by one watch of the bucket. A bus that does not answer, a lease lost and a key moved are now not-now like the store being away, so a merge whose plan could not be written is held and handed over again, never acknowledged. The start moves release_plan and batched_merge into the bucket, one kind, counted against both tables.

cmd/mesh-controller/upgrades.go

@@ -303,6 +303,16 @@ func notNow(err error) error {
 	return err
 }
 
+// unsaved marks the failure to save the plan a merge was added to, whatever it was: the merge is carried until that save
+// writes it (inventory.AddMerge), so the announcement is held and handed over again, never taken (novox/hq ADR 0407
+// decision 3, ADR 0408, issue 1092). The bus acknowledges it only after the save succeeded.
+func unsaved(err error) error {
+	if err == nil {
+		return nil
+	}
+	return fmt.Errorf("%w: the merge is not kept until its plan is saved: %w", link.ErrTryAgain, err)
+}
+
 // SourceMoved is the forge announcing a merge. It is put into the open batch (novox/hq ADR 0276), which
 // becomes one walk when its merge window closes: hearMerge, and cutBatchesHeld in batches.go.
 func (f following) SourceMoved(ctx context.Context, m link.SourceMoved) error {