Issue:
------
The `create` method (models.py) doesn't scale correctly
(with huge number of values) **when compute store no-readonly
field(s) are in values** (at most one).
It is due to the method `protecting` (api.py) which has a
bad complexity in the `create` situation
(list of (field, records) pairs as argument):

O(r²) with `r` = number of record containing the protected field:
List of `len(r)` send to `protecting`,
loop of this list (`r` factor), loop on fields (constant factor),
create a new frozen set with the previous one (`r` factor).

Fix:
----
Decrease the complexity to O(r) by creating a map of set of ids by
protected field which allows avoiding recreating a new frozenset
for each record (update with tuple of one inside => O(1)).

Performance gain:
----------
For a very simple model with only one compute store no-readonly field,
and all `create` `values` contains this field.
```
+--------------+---------------+---------------+---------------+----------------+
|   Batch ->   |     1000      |     10000     |    30000      |     80000      |
+--------------+---------------+---------------+---------------+----------------+
| Before (sec) | 0.083 ± 0.006 | 1.433 ± 0.091 | 8.764 ± 0.609 | 94.428 ± 3.021 |
| After (sec)  | 0.069 ± 0.003 | 0.706 ± 0.006 | 2.188 ± 0.076 | 5.875 ± 0.104  |
+--------------+---------------+---------------+---------------+----------------+
```