Keep a build node's pause in a pauses state of its own, so the steps state can drop its old keys without ending a pause

The steps state of the build modules filled to the bus's 64 MiB cap (27,370 keys of mesh-build-go,
none ever removed) and refused a passed check's end. Its keys get an age in the catalogue; the
pause key lived among them and would then have ended by itself. Puller.PausePut writes it to
delivery.PausesState instead; Put stays the fallback for a module not yet moved.

go/delivery/stepasks.go

@@ -535,9 +535,13 @@ func ParseAskStepKey(key string) (node, askID string, err error) {
 	return node, askID, nil
 }
 
-// PauseKey is the key, in a build module's StepsBucket, that keeps whether the module on node takes asks:
+// PausesState is the state a build module keeps its nodes' pause keys in. It has no age, unlike StepsState, whose keys
+// are per step and dropped after their age: a pause kept among the steps would end by itself.
+const PausesState = "pauses"
+
+// PauseKey is the key, in a build module's PausesState, that keeps whether the module on node takes asks:
 // `paused.<node>` (issue 1887's design limit 2: every holder pulls from one shared worker, so a node's pause is its holder
-// not fetching, kept here so a restart keeps it). A pause key is never an ask's step key: its second part is a node.
+// not fetching, kept so a restart keeps it). A pause key is never an ask's step key: its second part is a node.
 func PauseKey(node string) (string, error) {
 	if err := checkName("node", node); err != nil {
 		return "", err