Various updates to SqlServerSyntax are needed

[RachelAmbler]

Multiple issues: Things like next should be NVarChar(Max), Double quotes are used where [] should be, the ListDatabases` property needs to exclude system database, read only databases and unavailable databases. [] should be added in multiple other places to avoid potential errors.

I've already made these changes and I'll raise a PR for these later on.

[erikbra]

Mostly solved by #501 - please open another issue if you see anything else that should be changed. It's way easier to update the grate structure after #482 :)