Heavy refactor of the task system

Get rid of the class based task definition in favor of decorator-based
task declarations.

Doing so, we can get rid of core/tasks.py

Task names are explicitely set to keep compatibility with task
definitions in schedulers' database.

This also add debug statements at the beginning and end of each lister
task.

swh/lister/github/tasks.py

@@ -2,25 +2,52 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from swh.lister.core.tasks import (IndexingDiscoveryListerTask,
-                                   RangeListerTask,
-                                   IndexingRefreshListerTask, ListerTaskBase)
+import random
 
-from .lister import GitHubLister
+from celery import group
+
+from swh.scheduler.celery_backend.config import app
+from swh.scheduler.task import SWHTask
+
+from swh.lister.github.lister import GitHubLister
+
+GROUP_SPLIT = 10000
 
 
-class GitHubListerTask(ListerTaskBase):
-    def new_lister(self, *, api_baseurl='https://api.github.com'):
-        return GitHubLister(api_baseurl=api_baseurl)
+def new_lister(api_baseurl='https://api.github.com', **kw):
+    return GitHubLister(api_baseurl=api_baseurl, **kw)
 
 
-class IncrementalGitHubLister(GitHubListerTask, IndexingDiscoveryListerTask):
-    task_queue = 'swh_lister_github_discover'
+@app.task(name='swh.lister.github.tasks.IncrementalGitHubLister',
+          base=SWHTask, bind=True)
+def incremental_github_lister(self, **lister_args):
+    self.log.debug('%s, lister_args=%s' % (
+        self.name, lister_args))
+    lister = new_lister(**lister_args)
+    lister.run(min_bound=lister.db_last_index(), max_bound=None)
+    self.log.debug('%s OK' % (self.name))
 
 
-class RangeGitHubLister(GitHubListerTask, RangeListerTask):
-    task_queue = 'swh_lister_github_refresh'
+@app.task(name='swh.lister.github.tasks.RangeGitHubLister',
+          base=SWHTask, bind=True)
+def range_github_lister(self, start, end, **lister_args):
+    self.log.debug('%s(start=%s, end=%d), lister_args=%s' % (
+        self.name, start, end, lister_args))
+    lister = new_lister(**lister_args)
+    lister.run(min_bound=start, max_bound=end)
+    self.log.debug('%s OK' % (self.name))
+
+
+@app.task(name='swh.lister.github.tasks.FullGitHubRelister',
+          base=SWHTask, bind=True)
+def full_github_relister(self, split=None, **lister_args):
+    self.log.debug('%s, lister_args=%s' % (
+        self.name, lister_args))
+    lister = new_lister(**lister_args)
+    ranges = lister.db_partition_indices(split or GROUP_SPLIT)
+    random.shuffle(ranges)
+    group(range_github_lister.s(minv, maxv, **lister_args)
+          for minv, maxv in ranges)()
+    self.log.debug('%s OK (spawned %s subtasks)' % (self.name, len(ranges)))
 
 
-class FullGitHubRelister(GitHubListerTask, IndexingRefreshListerTask):
-    task_queue = 'swh_lister_github_refresh'