All 'Async' operations must return at least a Task object

I have noticed that I have some functions which are `*Async`, but do not return a `Task` object.  They must, as this is part of the async pattern.

An example can be found in [the NH 4.x transaction adapter](https://github.com/csf-dev/CSF.ORM/blob/master/CSF.ORM.NHibernate4/TransactionAdapter.cs).  This must be fixed.