Query: Support for outer apply and multi level binding

[smitpatel]

[ConditionalFact]
        public Task Query_compiler_concurrency()
        {
            const int threadCount = 50;

            var tasks = new Task[threadCount];

            for (var i = 0; i < threadCount; i++)
            {
                tasks[i] = Task.Run(
                    () =>
                    {
                        using (var context = CreateContext())
                        {
                            using ((from c in context.Customers
                                    where c.City == "London"
                                    orderby c.CustomerID
                                    select (from o1 in context.Orders
                                            where o1.CustomerID == c.CustomerID
                                                  && o1.OrderDate.Value.Year == 1997
                                            orderby o1.OrderID
                                            select (from o2 in context.Orders
                                                    where o1.CustomerID == c.CustomerID
                                                    orderby o2.OrderID
                                                    select o1.OrderID)))
                                .GetEnumerator())
                            {
                            }
                        }
                    });
            }

            return Task.WhenAll(tasks);
        }

Ignore concurrency part.
For the query, the inner-most collection references c but it is projecting list of ints. We convert collection of results into left join but since it is referencing outer elements which cannot be removed, we need to apply "outer apply"

[smitpatel]

Pulled in 3.0