luigid logging: Log less and change logger name (#1636)

I think it makes sense to log less, because currently I think INFO level
should not be verbose. In particular "No workers connected" message is
verbose and going to be said many more times than it will be actually
removed (when used with assistants).

Further, I think now is a sane time to change the name of the logger to
the "recommended" `getLogger(__name__)` style. Because PR
#1633 was just merged and it's less likely that people have
relied on the logger name prior to that.

luigi/scheduler.py

@@ -42,7 +42,7 @@
 from luigi.task_status import DISABLED, DONE, FAILED, PENDING, RUNNING, SUSPENDED, UNKNOWN
 from luigi.task import Config
 
-logger = logging.getLogger("luigi.server")
+logger = logging.getLogger(__name__)
 
 
 class Scheduler(object):
@@ -414,8 +414,8 @@ def update_status(self, task, config):
         # Mark tasks with no remaining active stakeholders for deletion
         if not task.stakeholders:
             if task.remove is None:
-                logger.info("Task %r has stakeholders %r but none remain connected -> might remove "
-                            "task in %s seconds", task.id, task.stakeholders, config.remove_delay)
+                logger.debug("Task %r has stakeholders %r but none remain connected -> might remove "
+                             "task in %s seconds", task.id, task.stakeholders, config.remove_delay)
                 task.remove = time.time() + config.remove_delay
 
         # Re-enable task after the disable time expires
@@ -531,7 +531,7 @@ def _prune_workers(self):
         remove_workers = []
         for worker in self._state.get_active_workers():
             if worker.prune(self._config):
-                logger.info("Worker %s timed out (no contact for >=%ss)", worker, self._config.worker_disconnect_delay)
+                logger.debug("Worker %s timed out (no contact for >=%ss)", worker, self._config.worker_disconnect_delay)
                 remove_workers.append(worker.id)
 
         self._state.inactivate_workers(remove_workers)
@@ -914,7 +914,7 @@ def dep_func(t):
 
             task = self._state.get_task(task_id)
             if task is None or not task.family:
-                logger.warn('Missing task for id [%s]', task_id)
+                logger.debug('Missing task for id [%s]', task_id)
 
                 # NOTE : If a dependency is missing from self._state there is no way to deduce the
                 #        task family and parameters.