Adds Task.request.headers
RPC Backend related improvements:

* reply_to is now used as a property

* correlation_id message property now set by amqp and rpc backends.

* CELERY_RESULT_PERSITENT is now None by default,
  and the actual default is decided by the result backend.

* amqp backend is persistent by default, rpc backend is not.

* rpc backend now uses the anon exchange to bypass routing.