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.

testdata/store-tests

@@ -149,3 +149,6 @@
 ./internal/inventory TestTheMoveCarriesTheBuildRequestsAndMarksTheNeverAskedToAskAgain
 ./internal/inventory TestTheMoveCarriesThePlansAndTheirMergesAndTheyReadTheSame
 ./cmd/mesh-controller TestAMergeWhosePlanCannotBeSavedIsHeldAndRecordedWhenHandedOverAgain
+./internal/inventory TestAnAssignmentIsAKeyOfTheBucketAndAnUnassignmentEndsIt
+./internal/inventory TestThePlansAreKeptInTheAssignmentsValuesAndNothingElseOfThemMoves
+./internal/inventory TestTheMoveCarriesTheAssignmentsAndWhatEachMachineWasSent