Poller methods within original class MBSProducer become module level
functions and are registered as Celery periodic tasks.
Code logging the size of fedmsg-hub queue are removed from log_summary.
process_open_component_builds is still kept there and not converted to a
periodic task.
There are some small refactor:
* do not format string in logging method call.
* reformat some lines of code doing SQLAlchemy database query to make
them more readable.
Signed-off-by: Chenxiong Qi <cqi@redhat.com>