Introducing a way to control lock modes when using the NHibernate saga persister

We're also changing the default mode from None to Upgrade since
that improves throughput when there is alot of concurrency on saga
instances. This also seems to prevent
https://nhibernate.jira.com/browse/NH-3023 from happening as well since
there will be much less rollbacks due to concurrency issues.
Closes #1342