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/migrations/0104-the-chain-moves-to-the-bus.sql

@@ -14,3 +14,8 @@ create table chain_moved (
     counts   jsonb       not null,
     largest  jsonb       not null
 );
+
+-- A seat's holder was an assignment by a foreign key; the assignments are keys of the bus once moved, so the controller says
+-- it (HoldSeat refuses a holder that is not assigned) and an unassignment takes the holding with it (Unassign), as the
+-- key's `on delete cascade` did.
+alter table seat_holding drop constraint seat_holding_node_module_fkey;