[13.0][FIX] ir.cron: check registry earlier to avoid corner case

[nilshamerlinck]

Description of the issue/feature this PR addresses:

• start instance with workers = 1 and max_cron_threads = 1
• create new db
• create 2 inactive ir.cron, cronSystem and cronAdmin:

[{"user_id":1,"interval_number":1,"interval_type":"minutes","active":false,"numbercall":-1,"state":"code","type":"ir.actions.server","name":"cronAdmin","model_id":1}, {"user_id":2,"interval_number":1,"interval_type":"minutes","active":false,"numbercall":-1,"state":"code","type":"ir.actions.server","name":"cronSystem","model_id":1}]

• install im_livechat
• activate cronSystem
• restart instance (to empty the cron worker's cache)
• cronSystem will run, and thus load the registry in the cron worker
• deactivate cronSystem
• uninstall im_livechat
• activate cronAdmin
• cronAdmin will fail to run with this traceback:

2020-11-02 16:21:23,695 46 ERROR v13c_cron odoo.addons.base.models.ir_cron: Unexpected exception while processing cron job {'id': 16, 'ir_actions_serv
er_id': 176, 'cron_name': 'cronAdmin', 'user_id': 2, 'active': True, 'interval_number': 1, 'interval_type': 'minutes', 'numbercall': -1, 'doall': None
, 'nextcall': datetime.datetime(2020, 11, 2, 16, 7, 45), 'lastcall': datetime.datetime(2020, 11, 2, 16, 7, 26), 'priority': 5, 'create_uid': 2, 'creat
e_date': datetime.datetime(2020, 11, 2, 8, 0, 45, 442940), 'write_uid': 2, 'write_date': datetime.datetime(2020, 11, 2, 16, 20, 50, 669605)}
Traceback (most recent call last):
  File "/opt/odoo/odoo/odoo/api.py", line 745, in get
    value = self._data[field][record._ids[0]]
KeyError: 2

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/odoo/odoo/odoo/fields.py", line 1000, in __get__
    value = env.cache.get(record, self)
  File "/opt/odoo/odoo/odoo/api.py", line 751, in get
    raise CacheMiss(record, field)
odoo.exceptions.CacheMiss: ('res.users(2,).tz', None)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/odoo/odoo/odoo/api.py", line 745, in get
    value = self._data[field][record._ids[0]]
KeyError: 2

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/odoo/odoo/odoo/fields.py", line 1000, in __get__
    value = env.cache.get(record, self)
  File "/opt/odoo/odoo/odoo/api.py", line 751, in get
    raise CacheMiss(record, field)
odoo.exceptions.CacheMiss: ('res.users(2,).partner_id', None)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/odoo/odoo/odoo/addons/base/models/ir_cron.py", line 237, in _process_jobs
    registry[cls._name]._process_job(job_cr, job, lock_cr)
  File "/opt/odoo/odoo/odoo/addons/base/models/ir_cron.py", line 138, in _process_job
    now = fields.Datetime.context_timestamp(cron, datetime.now())
  File "/opt/odoo/odoo/odoo/fields.py", line 1786, in context_timestamp
    tz_name = record._context.get('tz') or record.env.user.tz
  File "/opt/odoo/odoo/odoo/fields.py", line 1024, in __get__
    self.compute_value(recs)
  File "/opt/odoo/odoo/odoo/fields.py", line 1109, in compute_value
    records._compute_field_value(self)
  File "/opt/odoo/odoo/odoo/models.py", line 3968, in _compute_field_value
    field.compute(self)
  File "/opt/odoo/odoo/odoo/fields.py", line 567, in _compute_related
    values = [first(value[name]) for value in values]
  File "/opt/odoo/odoo/odoo/fields.py", line 567, in <listcomp>
    values = [first(value[name]) for value in values]
  File "/opt/odoo/odoo/odoo/models.py", line 5689, in __getitem__
    return self._fields[key].__get__(self, type(self))
  File "/opt/odoo/odoo/odoo/fields.py", line 2329, in __get__
    return super().__get__(records, owner)
  File "/opt/odoo/odoo/odoo/fields.py", line 1007, in __get__
    recs._fetch_field(self)
  File "/opt/odoo/odoo/odoo/models.py", line 2974, in _fetch_field
    self._read(fnames)
  File "/opt/odoo/odoo/odoo/addons/base/models/res_users.py", line 378, in _read
    super(Users, self)._read(fields)
  File "/opt/odoo/odoo/odoo/models.py", line 3046, in _read
    cr.execute(query_str, params)
  File "/opt/odoo/odoo/odoo/sql_db.py", line 173, in wrapper
    return f(self, *args, **kwargs)
  File "/opt/odoo/odoo/odoo/sql_db.py", line 250, in execute
    res = self._obj.execute(query, params)
psycopg2.ProgrammingError: column res_users.livechat_username does not exist
LINE 1: ...out_of_office_message" as "out_of_office_message","res_users...

This happens because:

• the uninstallation of im_livechat changed the res_users schema (dropped livechat_username)
• but the res.users record for admin is loaded with the old registry, because the signaling check only happens later in the _callback() part.

This PR makes sure the registry check is done before starting to _process_job().

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[lathama]

@nilshamerlinck knowing that 13 is no longer updated, is this PR still needed?