Change SQL Server dialect to use COUNT_BIG for count

To have a count for Int32 and register count_big for Int64 mean a breaking change.
Now using count in HQL mean count_big in MSSQL.

HQL function are supported in NH2.0.
We only need to change the MsSql2000Dialect

I won't be able to change it for 1.2.0.CR1 because custom SQL function support hasn't been implemented yet. Leaving this for later.

This was changed for compatibility with Hibernate. HQL COUNT should now return Int64 on all databases. I will change COUNT for SQL Server to use COUNT_BIG(). You can revert to the original behavior by registering the original count function in the dialect:

Dialect dialect = sessionFactory.Dialect.RegisterFunction("count", new ClassicCountFunction());