Zombie check before transaction commit or rollback

[alex-kulakov]

Suggested fix #77

The fix is applied only for SQL Server. The PR checks whether transaction is became zombie before commit or rollback. This happens when some exception appeared and rolled back existing transaction (in this case an attempt to rollback to save point when DbReader is active).

User will have original exception (e.g. inability to perform rollback to save point due to open reader) instead of overlapping exception from outermost TransactionScope.Dispose()

[AlexUstinov]

Is this really actual only for the MSSQL provider?

[alex-kulakov]

The #77 issue appears only for MS SQL. I have tried to run the tests on other supported RDBMS.

If we speak about checking connection in general then I couldn't find any error with similar symptoms in other RDBMSs to add it as a test. Having DbTransaction.Connection set to null when connection is corrupted is not general behavior so I think we will fix it later if something shows up for other providers.