NHibernate Saga persister does not work with Oracle

The issue is that the Oracle driver does not support both TransactionScope and NH transaction, see http://stackoverflow.com/q/6870901/90882 for more info.
So we now checking if there is currently an active ambient transaction and if there is we do not create a NH transaction.
We also disable AutoFlush and control Flush manually.

Still to do:
- [x] fix broken acceptance tests
- [x] code review