Collect affected exchanges more efficiently

When an incoming exchange of an order cycle changes, the ProductsCache
queries all affected outgoing exchanges to update them. It was creating
a big collection of exchanges with duplicates and then calling `uniq`.
That call was hitting a custom implementation of `eql?` which is very
inefficient. And since `Exchange.eql?` is ignoring the order cycle id,
it was probably filtering too many exchanges from the collection.

Fixed bug: If two order cycles sell exactly the same variants to the
same shop, the two outgoing exchanges are seen as equal. When the
variants change, ProductsCache would only update one of those two
exchanges, leaving one order cycle out of sync. This case is very rare.
It only happens if there is a shop with two active order cycles selling
exactly the same.

The new uniqueness test looks only at the attributes that are later used
to refresh the cache. I measured a page speed improvement from 90
seconds to 3 seconds (30 times faster).

lib/open_food_network/products_cache.rb

@@ -170,7 +170,8 @@ module OpenFoodNetwork
           incoming_exchange.order_cycle,
           incoming_exchange.variant_ids
         )
-      end.flatten.uniq
+      end.flatten.uniq { |ex| [ex.receiver_id, ex.order_cycle_id]}
+      # Comparing only on these two ids is much faster.
     end
     private_class_method :outgoing_exchanges_affected_by