Convert the Poller to be Celery periodic tasks
    
    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>