Nullify the disposing components to prevent memory leaks due circular reference.

Author: hazzik

src/NHibernate/Impl/AbstractSessionImpl.cs

@@ -330,6 +330,7 @@ protected internal void SetClosed()
 			{
 				if (TransactionContext != null)
 					TransactionContext.Dispose();
+				TransactionContext = null;
 			}
 			catch (Exception)
 			{

src/NHibernate/Transaction/AdoNetWithDistributedTransactionFactory.cs

@@ -94,7 +94,7 @@ public class DistributedTransactionContext : ITransactionContext, IEnlistmentNot
 		{
 			public System.Transactions.Transaction AmbientTransation { get; set; }
 			public bool ShouldCloseSessionOnDistributedTransactionCompleted { get; set; }
-			private readonly ISessionImplementor sessionImplementor;
+			private ISessionImplementor sessionImplementor;
 			public bool IsInActiveTransaction;
 
 			public DistributedTransactionContext(ISessionImplementor sessionImplementor, System.Transactions.Transaction transaction)
@@ -178,6 +178,8 @@ public void Dispose()
 			{
 				if (AmbientTransation != null)
 					AmbientTransation.Dispose();
+				AmbientTransation = null;
+				sessionImplementor = null;
 			}
 		}
 	}