walk back SQL Server language a bit re: insertmanyvalues

walk back SQL Server language a bit re: insertmanyvalues

In #9618 we both can look to re-enable insertmanyvalues
for SQL Server, and also likely *disable* its use for the
ORM unit of work specifically, since that's really where the
only problem is, and it will likely be for all dialects,
not just SQL Server.   An approach using sentinel columns will
be rolled out for the unit of work use case.

Change-Id: I3358e30839491769db95b4ac042a661271df3929
References: #9618
References: #9603