Added resource string for further use to indicate that a parent transaction is completed before child one.

Allowed several nested ambient transactions to be processed. Main scenario is when one transaction with TransactionScopeOption.RequiresNew is created inside another one.

Added two unit tests to check nested ambient scenarios - one for normal flow when both transaction are committed, another for a case when child transaction is committed, and parent is not.

…_Ambient_Transaction

"Count" in this case works better than "Any"