Remove some invalid SQL Server types and correct others

[beppler]

Some types returned on SQLServer dialect did not exist and others could be optimized, but will be only compatible with SQL Server 2008 or newer.

The biggest difference are:

• DbType.Binary that now returns VARBINARY instead of BINARY,
• DbType.AnsiString that now returns VARCHAR(MAX) for precision greater than 8000
• DbType.AnsiStringFixedLength that now returns CHAR, but only for precision between 0 and 8000
• DbType.Date that now returns DATE
• DbType.DateTime2 has been added
• DbType.String that now returns NVARCHAR(MAX) for precision greater than 4000
• DbType.StringFixedLenght that now return NCHAR, but only for precision between 0 and 4000

For types that are not supported the error message explains that it could be because of type and precision.

[andrecarlucci]

Hi @beppler ,

I changed these mappings so many times that I would like to do it slowly for now on. Here is what we have today:

https://github.com/SharpTools/sharpdata/wiki/DbTypes-x-SqlServer

Please, create "one issue per change" of type justifying it and we could collaboratively talk about it. Thanks!

[beppler]

Ok, I will close this pull request and then I will open the issues, one for each type. The types that I used are based on SQL Server Data Type Mappings from Microsoft.

[beppler]

Issues #8, #9, #10, #11, #12, #13 and #14 opened.

So I´m closing this pull request.