[FIX] hr_holidays: fasten creation of leaves by company

When creating a leave, we usually check/create activities for the
concerned users, this is fine when creating one leave.
However it starts to become a problem when trying to create leaves in
batch using the by company or by department holiday_type as the number
of leaves created in batch can become quite big.

Now when we create leaves in batch, we do not check for the activities,
which will fasten a lot the creation.

Another problem is that when someone defined a leave on the very day you
want to create such a request, then it will crash, this can lead to
waste of time and resources.

Now we raise an error before trying to create the leaves in case of
conflict.

NB: Tested with +- 320 records, took 20 seconds instead of more than 10 minutes.

Task: 1934383

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closes odoo/odoo#30625