Add left join support for Linq query provider

[maca88]

Fixes: #864

This PR provides two additional ways to do a left join:

• Query syntax

from c in db.Customers
  join o in db.Orders on c.CustomerId equals o.Customer.CustomerId into orders
  from o in orders.DefaultIfEmpty()
  select o;

• Extension method

db.Orders
  .LeftJoin(db.Orders, x => x.OrderId, x => x.OrderId - 1, (order, order1) => new { order, order1 })
  .Select(x => new { First = x.order, Second = x.order1 })
  .ToList();

[hazzik]

Please resolve conflicts

[maca88]

Rebased and commented the Linq to Objects query.

[maca88]

Rebased.

[hazzik]

@maca88 I'm trying to understand why we need booth NhJoin (which has a flag IsInner) and NhOuterJoin?

[maca88]

At first I tried to use NhJoin but it didn't worked out as it implements IFromClause, which requires fromExpression parameter that we don't have in this case. Ideally, if JoinClause would have a property whether it is an inner or outer join we wouldn't need NhOuterJoinClause, which would simplify the code and avoid registering both joins in QuerySourceIdentifier:

nhibernate-core/src/NHibernate/Linq/Visitors/QuerySourceIdentifier.cs

Lines 63 to 64 in 0132e4c

	_namer.Add(outerJoinClause);
	_namer.Add(outerJoinClause.JoinClause, _namer.GetName(outerJoinClause));

I was also thinking about tracking which JoinClause is an outer join in order to avoid having a wrapper, but the problem is that JoinClause can be cloned when rewriting the query model, which would probably ends up with a more complex solution.

[bahusoid]

@hazzik Do you still have some pending questions/plans with reviewing this PR?

[bahusoid]

Oups.. Something is not right... LINQ tests are failing on master after merge

[hazzik]

There is some interference with #2324.