Share the controller's stores with its take-ins and cap take-ins at 16

At 16:07:36 the builder rewrote about 120 done keys at once. Each woke a
take-in of its own with no limit, and each take-in read the rest of the
mesh through restOfTheMeshOf, which opened a licences and an identity
pool per call. novox's store ran out of connections at 16:08 and refused
everyone, the controller's retries keeping it full.

- The serving controller lends its stores by inventory (shareStores);
  restOfTheMeshOf reads on them instead of opening pools per call, and a
  verb run as its own process still opens its own. stores opens each
  store once under a lock, since readers now share it. serve uses the
  shared identity store instead of a second pool.
- The watch's take-ins and the start's look share one cap of 16 take-ins
  at once (takeInSlots); a take-in waiting for its next try holds none.
- store.Ready backs off on a full store (SQLSTATE 53300), doubling its
  wait up to 5 s, and says once that the store is full.

Issues 1945 and 1277.

cmd/mesh-controller/compose_sdk.go

@@ -326,9 +326,13 @@ func restOfTheMesh(ctx context.Context, open *stores, shelf map[string]catalogue
 	return world, nil
 }
 
-// restOfTheMeshOf is restOfTheMesh for a caller that holds the inventory alone: the other stores the Input reads are opened for
-// the call and let go of again.
+// restOfTheMeshOf is restOfTheMesh for a caller that holds the inventory alone: on the serving controller's stores where it
+// lends them (sharedStores), never a pool per call; elsewhere (a verb run as its own process) the other stores the Input
+// reads are opened for the call and let go of again.
 func restOfTheMeshOf(ctx context.Context, inv *inventory.Inventory, shelf map[string]catalogue.Manifest, exclude string) (catalogue.World, error) {
+	if shared, ok := sharedStoresOf(inv); ok {
+		return restOfTheMeshOn(ctx, shared, shelf, exclude)
+	}
 	open := &stores{inventory: inv}
 	defer func() {
 		if open.licences != nil {
@@ -338,9 +342,12 @@ func restOfTheMeshOf(ctx context.Context, inv *inventory.Inventory, shelf map[st
 			open.identity.Close()
 		}
 	}()
-	return restOfTheMesh(ctx, open, shelf, exclude)
+	return restOfTheMeshOn(ctx, open, shelf, exclude)
 }
 
+// restOfTheMeshOn is restOfTheMesh; a variable so a test reads which stores it is handed.
+var restOfTheMeshOn = restOfTheMesh
+
 // sayFound says the lines a composition found while composing, once each: a machine left out of the rest of the mesh, a seat
 // that two claimants could take, the bus's users left out for want of a credential.
 func sayFound(ctx context.Context, d compose.Declared) {