WL#3686: Avoid read before PK/UK update/deletes without expressions

When performing a primary key or unique key update
it is almost never necessary to read the record
before the update. The exception to this is when the
primary key is updated since then a delete followed
by an insert is required and this must be prepared by
reading the record.

It's especially easy to implement this for primary key
or unique key updates where the readset is empty where
only constants are assigned to the fields.

For DBT2 this will remove around 8 primary key lookups
per transaction so it's a very significant performance
improvement.