Assemble merges in a rolling window and walk each batch once (hq ADR 0276, issue 362)

Every merge opened a walk and the next merge of the branch superseded it: two
catalogue merges 18 s apart left a walk no delivery held, and the operator
started it by hand 58 minutes later. A merge now joins the open batch, kept in
the store (migration 0089), which is cut into one walk when no merge came for
merge-window (90 s) or at merge-window-at-most (10 min): one commit per
repository, the latest of its branch, with every file the batch's merges
changed. One walk at a time; a started walk is never superseded, a waiting one
is folded into the next. The walk names every merge it answers on the wire
(delivery.merges, taken_over_by, batch). A failed walk walks its earlier merges
alone, newest first, until one is delivered. A delivery group's order becomes
tier edges inside the walk. plans shows the batch assembling; S18 and S19
bound its waits; S16 names the merges a waiting walk answers.

cmd/mesh-controller/push.go

@@ -182,6 +182,9 @@ func serve(ctx context.Context) (err error) {
 	// Open plans move on a timer as well as on outcomes (novox/hq ADR 0162): a tier waiting for
 	// machines to report moves when they have, and a plan left by a replaced controller resumes.
 	go planTicker(ctx, open)
+	// And the merge window (novox/hq ADR 0276): a batch is cut into its walk when the window closes, which no
+	// merge or outcome says.
+	go batchCutter(ctx, open)
 	// And the pending assignments settled on a tick of their own (novox/hq ADR 0261): made once their module
 	// is registered, ended with why when its build will not register it, raised and cleared as conditions.
 	// Never by a read.