`_add_missing_default_values` is made to work in batch.

Models with a sequence can take advantage of that to increase the sequence for
each new record in the batch.

Before this commit, all records created in a batch would have the same sequence
if no sequence was provided to `create`.

The new sequence is computed after the parent `_add_missing_default_values`
instead of before (or instead of in `create`) to keep the possibility to have
the default value coming from other means, as computed in `default_get`.

The opportunity is also taken to change the definition of some defaults methods
that would lead to the execution of queries, because those queries would be done
for each record in the batch.

This is typically the case for defaults that use `ref` which does an `exists`
(as opposed to `xmlid_to_res_model_res_id`) or it is obviously also the case for
the defaults doing a `search` directly.

Indeed the result of these queries would then either be read or be added as a
many2one, both of which will already check the existence of the resource.

The focus of this commit was on products and partners, but the same logic can be
applied to other models if they have such defaults and the performance of their
batch create is important.