Inforigami.Regalo.SqlServer: Support for externally managed connection/transaction

[davecluderaythinventory]

Currently

• Regalo manages its own SqlConnection.
• Regalo also manages the transaction (using TransactionScope).

With this PR

• If the store is created using a connection string, then Regalo continues to manage its own connection and transaction (although I have switched it to use an explicit ADO.NET transaction, since it made the implementation easier).
• Alternatively, the store can be created with a factory delegate that returns a new interface, ISqlSession. This exposes a DbConnection and DbTransaction, and allows for scenarios where Regalo needs to participate in an externally managed explicit transaction (such as that provided by NServiceBus.Persistence.Sql).

Additional Notes

• I chose to expose DbConnection and DbTransaction since it's what NServiceBus.Persistence.Sql (our use case) exposes itself, plus it wasn't much extra work (and technically might give a bit more flexibility if the Microsoft.Data.SqlClient library ever gets replaced again).
• There are two commits in this PR, and it might be easiest to review each in turn.