python/pgq update

python/pgq update

- remove the obsolete setconsumer stuff
- New CascadedConsumer / CascadedWorker classes,
  that are based on regular pgq.Consumer

- move RemoteConsumer / SerialConsumer out of pgq/consumer.py

pgq.Consumer:
- rename conf params and instance variables:
  pgq_queue_name -> queue_name
  pgq_consumer_id -> consumer_name
- disable automatic registration on the queue,
  now script needs to be called with switches --register / --unregister
- drop code to read from multiple-queues at once

pgq.ticker:
- drop event seq tracking code, this is now done in db