Improve TransactionScope settings options

[kbaley]

This issue was transferred from a private repository

Only one isolation level seems appropriate for usage with TransactionScope, i.e. ReadCommitted. Different options caused issues when using Outbox with connection pooling. E.g. https://github.com/Particular/NServiceBus.SqlServer/issues/310.

So this configuration setting needs improvements, but the changes will be (potentially) breaking, so should be released in major versions, and should be consistent across transports (MSMQ transport also supports that setting).

Suggested plan of action

• [ ] Determine which isolation levels should be supported or if we even need that setting at all or should it be completely deprecated (as suggested in https://github.com/Particular/NServiceBus.SqlServer/issues/312)
• [ ] Implement changes in SQL Server transport
• [ ] Document the reasoning and changes here