The main classes of partners and users support batch creation, but
the majority of their overrides doesn't support creation in batch.

Adapting those overrides to support records creation in batch shows
great performance gains:

* On `res_partner` : 2 to 3 times faster
* On `res_users`, with the inherited `res_partner` created in batch:
	up to 10 times faster.

Tests done with 500 to 4k records:

* `res_partner` with only a name provided
* `res_users` with a name and login

X-original-commit: 9c3c5f161580039c1fe50f68acac808c18817997