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/recordmirror/mirror.go

@@ -325,6 +325,10 @@ func validKey(key string) error {
 	return nil
 }
 
+// ChangedFields names the top-level fields whose values differ between two encoded objects, sorted; nil when either is
+// not an object: what an event of a changed key says of it.
+func ChangedFields(before, after []byte) []string { return changedFields(before, after) }
+
 // changedFields names the top-level fields whose values differ between two encoded objects, sorted; nil
 // when either is not an object.
 func changedFields(before, after []byte) []string {