Fixing use of "bigint" and "decimal" for pure integer types.

See https://github.com/silverstripe/silverstripe-mssql/issues/14 for
more info.

"bigint" doesn't work for PHP 32-bit, as the integer could get too big
for PHP to be able to interpret the value as an integer type. In that
case it would try to approximate it as a float, but that could lead to
a bad assumption for identity columns.

For compatibility, stick with standard "int" type which has a maximum
value of 2147483647.

Note that MySQLAdapter already has this maximum, so changing it to this
keeps this database adapter consistent with MySQL.

I'll open another PR against master, as the structure is quite different
now that the database adapters have changed for SS 3.2.