Set update/delete modes on fixture.

In SQLServer the SET NULL mode is used by default, and this mode cannot
work with not null fields.
Restart identity columns in SQLServer if they exist.

Since SQLServer won't let us truncate tables with foreign keys we can
use delete from and restart the identity sequences.
Fix up double truncation causing SQLServer to do silly things.
Merge pull request #4548 from cakephp/sqlserver-fix

Sqlserver fix