Impacted versions:
9.0, 10.0, 11.0 (Maybe 8.0)
I don't know if it is really an expected behaviour for odoo but I like to expose it

Steps to reproduce:

1. Open an ir.cron record and change it to:
 - interval: minutes
 - next execution date: now + 1 minute

2. Open the odoo server log

Note: I'm using workers=3 but I don't know if there is possible to reproduce it without workers.

Current behavior:

You will see for each cron execution the following output
```txt
...
2018-06-01 16:18:33,432 20686 INFO 333561-11-0-42aff9-all odoo.modules.registry: Invalidating all model caches after database signaling.
...
2018-06-01 16:17:21,527 20687 INFO 333561-11-0-42aff9-all odoo.modules.loading: loading 1 modules...
2018-06-01 16:17:21,535 20687 INFO 333561-11-0-42aff9-all odoo.modules.loading: 1 modules loaded in 0.01s, 0 queries
2018-06-01 16:17:21,594 20687 INFO 333561-11-0-42aff9-all odoo.modules.loading: loading 206 modules...
2018-06-01 16:17:21,718 20687 INFO 333561-11-0-42aff9-all odoo.modules.loading: 206 modules loaded in 0.12s, 0 queries
2018-06-01 16:17:21,989 20687 INFO 333561-11-0-42aff9-all odoo.modules.loading: Modules loaded.
```

Expected behavior:
 - Review if the method that run from the cron really requires a invalidate cache and delete all registries.
 - Disable "minutes" for cron interval because all cache are deleted then is not valid use them.

Video/Screenshot link (optional):

 - [![IMAGE ALT TEXT HERE](https://img.youtube.com/vi/Be4FIeQjgjk/0.jpg)](https://youtu.be/Be4FIeQjgjk)
openerp/modules/registry.py
""" Return the size of the registry. """
        return len(self.models)
    def __iter__(self):
        """ Return an iterator over all model names. """
        return iter(self.models)
    def __getitem__(self, model_name):
        """ Return the model with the given name or raise KeyError if it doesn't exist."""
        return self.models[model_name]
    def __call__(self, model_name):
        """ Same as ``self[model_name]``. """
        return self.models[model_name]
    @lazy_property
    def model_cache(self):
        return RegistryManager.model_cache
    @lazy_property
    def pure_function_fields(self):
        """ Return the list of pure function fields (field objects) """
        fields = []
        for mname, fnames in self._pure_function_fields.iteritems():
            model_fields = self[mname]._fields
            for fname in fnames:
                fields.append(model_fields[fname])
        return fields
    @lazy_property
    def field_sequence(self):