Keep assignments in the assignments bucket and move the rows there at start (issue 1093)

Once chain_moved records the kind, Assign, Unassign and Assigned write and read the keys <node>.<module>, and every join of the store that read assignments is made from the bucket: where a module runs, what is behind, what a machine awaits, a given value due for replacement, taking a module, and whether a module may be forgotten (refused while a standing assignment names it). An unassignment ends the key with a marker and keeps its earlier values, as it removed the row; no ended or pending mark is written yet. The generation rises with each change, and a seat held by an assignment goes with it: the foreign key from seat_holding is dropped by migration 0104 and the controller says both. The provisioner's plan is kept in the values by a pass of the tick that writes that one field by compare-and-set; the assignment-changed event is said from what the bucket holds, whoever wrote it. The start moves the rows with what each machine was last sent, keeping a plan the mirror put in a value.

internal/chain/bus.go

@@ -76,6 +76,16 @@ func (b busBucket) Update(ctx context.Context, key string, value []byte, revisio
 	return rev, err
 }
 
+func (b busBucket) Remove(ctx context.Context, key string, revision uint64) error {
+	// A delete marker, never a purge: the key's earlier values stay in the bucket's history.
+	err := b.kv.Delete(ctx, key, jetstream.LastRevision(revision))
+	var apiErr *jetstream.APIError
+	if errors.As(err, &apiErr) && apiErr.ErrorCode == jetstream.JSErrCodeStreamWrongLastSequence {
+		return ErrMoved
+	}
+	return err
+}
+
 func (b busBucket) Keys(ctx context.Context) ([]string, error) {
 	keys, err := b.kv.Keys(ctx)
 	if errors.Is(err, jetstream.ErrNoKeysFound) {
@@ -126,6 +136,9 @@ func (b busBucket) History(ctx context.Context, key string) ([]Entry, error) {
 	}
 	out := make([]Entry, 0, len(all))
 	for _, e := range all {
+		if e.Operation() != jetstream.KeyValuePut {
+			continue
+		}
 		out = append(out, entryOf(e))
 	}
 	return out, nil