In v16 (and it's still the case in master) fields default values are not batched. They are added one by one for each vals dict in `_prepare_create_values`. The main reason for that is that a given default could be a callable returning a randomly generated value.

Still, that can be a bottleneck when batch creating records, during imports for instance. That's especially true for a model such as account.move, which is already pretty heavy to create. When the sale module is installed, a new field `team_id` is added. This field is a computed-stored field with a callable set as default. Because the default attr is defined, the ORM skips the call to the compute method and calls the default function instead when the team_id param is not given upon creation. That means that for each account.move, `_get_default_team_id` is called. Since the purpose of a compute function is already to give default values to record, removing the default callable allows to benefit from the batching of compute functions.

#### speedup

Test database with account_accountant and sale.
Trying to create batches of account.moves

| Batch size | Before PR | After PR |
|:----------:|:---------:|:--------:|
|     1      |    70ms   |    58ms  |
|     50     |    150ms  |    100ms |
|     100    |    275ms  |    188ms |
|     250    |    660ms  |    450ms |
|     1000   |    2.43s  |    1.75s |
|     3000   |    7.55s  |    5.33s |

Average speedup: 68%.




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Forward-Port-Of: odoo/odoo#160313
Forward-Port-Of: odoo/odoo#157887