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/service/server.py
_logger.debug("WorkerCron (%s) polling for jobs", self.pid)
        db_names = self._db_list()
        if len(db_names):
            self.db_index = (self.db_index + 1) % len(db_names)
            db_name = db_names[self.db_index]
            self.setproctitle(db_name)
            if rpc_request_flag:
                start_time = time.time()
                start_rss, start_vms = memory_info(psutil.Process(os.getpid()))
            import openerp.addons.base as base
            base.ir.ir_cron.ir_cron._acquire_job(db_name)
            openerp.modules.registry.RegistryManager.delete(db_name)
            
            if len(db_names) > 1:
                openerp.sql_db.close_db(db_name)
            if rpc_request_flag:
                run_time = time.time() - start_time
                end_rss, end_vms = memory_info(psutil.Process(os.getpid()))
                vms_diff = (end_vms - start_vms) / 1024
                logline = '%s time:%.3fs mem: %sk -> %sk (diff: %sk)' % \
                    (db_name, run_time, start_vms / 1024, end_vms / 1024, vms_diff)
                _logger.debug("WorkerCron (%s) %s", self.pid, logline)
            self.request_count += 1
            if self.request_count >= self.request_max and self.request_max < len(db_names):
                _logger.error("There are more dabatases to process than allowed "
                              "by the `limit_request` configuration variable: %s more.",
                              len(db_names) - self.request_max)
        else:
            self.db_index = 0
    def start(self):