Raise a check's store without durability and remove what earlier holders left (hq issue 306)

On the control node the store-bound packages of the controller's suite ran
five to seven times slower than on any other holder, and every controller
check that landed there ran past the suite's thirty minutes: a throwaway
store flushing to a disk the mesh's own store, bus and forge keep busy.
A store that lives for one check needs no crash safety.

A holder recreated mid-check left the check's store and bus running, and
the redelivery went to another machine, so nothing removed them: eleven
pairs across four machines. A starting holder has taken nothing, so every
container labelled with an ask of the seat is an earlier holder's.

internal/builder/check.go

@@ -346,7 +346,7 @@ func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry
 	}()
 	labelled := Labelled(run, spec.ID)
 	store, err := throwaway(ctx, labelled, run, spec.ID+"-store", StoreImage(f.Versions.Store), 5432,
-		[]string{"-e", "POSTGRES_PASSWORD=check"}, nil)
+		[]string{"-e", "POSTGRES_PASSWORD=check"}, StoreSettings)
 	if err != nil {
 		return CheckVerdict{}, err
 	}
@@ -848,6 +848,16 @@ func StoreImage(version string) string {
 	return "postgres:" + major + "-alpine"
 }
 
+// StoreSettings are the throwaway store's server settings: **no durability** (novox/hq issue 306). The
+// store lives for one check and is removed after it, so nothing it writes has to survive a crash —
+// and a test suite that creates a database per test asks the disk to flush thousands of times. On a
+// machine whose disk the mesh's own store, bus and forge already keep busy — the control node — each
+// flush waited its turn: the store-bound packages of the controller's suite ran five to seven times
+// slower there than on any other holder, while those that touch no store ran alike, and its check ran
+// past the suite's thirty minutes every time it landed there. The release stays the mesh's; only what
+// a crash would need is switched off.
+var StoreSettings = []string{"-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"}
+
 // BusImage is the bus a check stands on: the release the mesh's bus server runs.
 func BusImage(version string) string {
 	v := strings.TrimPrefix(strings.TrimSpace(version), "v")