replace celery task decorators with a kombu-based publisher

this commit implements the bulk of `awx-manage run_dispatcher`, a new
command that binds to RabbitMQ via kombu and balances messages across
a pool of workers that are similar to celeryd workers in spirit.
Specifically, this includes:

- a new decorator, `awx.main.dispatch.task`, which can be used to
  decorate functions or classes so that they can be designated as
  "Tasks"
- support for fanout/broadcast tasks (at this point in time, only
  `conf.Setting` memcached flushes use this functionality)
- support for job reaping
- support for success/failure hooks for job runs (i.e.,
  `handle_work_success` and `handle_work_error`)
- support for auto scaling worker pool that scale processes up and down
  on demand
- minimal support for RPC, such as status checks and pool recycle/reload

awx/main/dispatch/reaper.py

@@ -0,0 +1,46 @@
+from datetime import timedelta
+import logging
+
+from django.db.models import Q
+from django.utils.timezone import now as tz_now
+from django.contrib.contenttypes.models import ContentType
+
+from awx.main.models import Instance, UnifiedJob, WorkflowJob
+
+logger = logging.getLogger('awx.main.dispatch')
+
+
+def reap_job(j, status):
+    j.status = status
+    j.start_args = ''  # blank field to remove encrypted passwords
+    j.job_explanation += ' '.join((
+        'Task was marked as running in Tower but was not present in',
+        'the job queue, so it has been marked as failed.',
+    ))
+    j.save(update_fields=['status', 'start_args', 'job_explanation'])
+    if hasattr(j, 'send_notification_templates'):
+        j.send_notification_templates('failed')
+    j.websocket_emit_status(status)
+    logger.error(
+        '{} is no longer running; reaping'.format(j.log_format)
+    )
+
+
+def reap(instance=None, status='failed'):
+    '''
+    Reap all jobs in waiting|running for this instance.
+    '''
+    me = instance or Instance.objects.me()
+    now = tz_now()
+    workflow_ctype_id = ContentType.objects.get_for_model(WorkflowJob).id
+    jobs = UnifiedJob.objects.filter(
+        (
+            Q(status='running') |
+            Q(status='waiting', modified__lte=now - timedelta(seconds=60))
+        ) & (
+            Q(execution_node=me.hostname) |
+            Q(controller_node=me.hostname)
+        ) & ~Q(polymorphic_ctype_id=workflow_ctype_id)
+    )
+    for j in jobs:
+        reap_job(j, status)