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/inventory/nodes.go

@@ -1285,6 +1285,11 @@ func (i *Inventory) RecordSentUnder(ctx context.Context, node, digest string, bu
 // read from: every change a push carries happened after the last push, and an assignment is the one
 // that says when.
 func (i *Inventory) AwaitingSince(ctx context.Context, name string) (time.Time, error) {
+	if _, on, err := i.ChainReads(ctx, KindAssignments); err != nil {
+		return time.Time{}, err
+	} else if on {
+		return i.chainAwaitingSince(ctx, name)
+	}
 	var since time.Time
 	err := i.store.Pool().QueryRow(ctx,
 		`select coalesce(