source-sql-server-batch: new batch connector in line with the others

[willdonnelly]

Exactly what it says on the tin. This should probably be a copy-paste of source-postgres-batch with the XMIN logic stripped out, and appropriate tweaks to the connection setup and discovery logic to make everything work nicely. Slap a bunch of datatype and various edge case tests on it and we're done!

One thing we might want to consider: SQL Server has no direct equivalent to the PostgreSQL xmin or Oracle ORA_ROWSCN system columns, but it does have a column type ROWVERSION which does something very similar when it's present as a column on a particular table (you just have to have added it yourself, it's not there by default). We should consider whether it's worth the effort of detecting when a source table has a column of that type and using it as the cursor automatically, versus just defaulting to full-refresh behavior.