disco core: avoid creating an instance of worker just for reading blobs
disco core: remove the Params class from disco core

This has been deprecated for a while. This might break some of the clients that
should switch to using the new import location
disco worker: rename ClassicFile to StreamCombiner

The old name did not make much sense.
disco core: remove the classic_iterator alias for result_iterator

The name result_iterator is by no means accurate, but it is widely used by the
the disco jobs. In the future, it should be renamed to something like
disco_iterator.
Merge pull request #573 from pooya/core_cleanup

Core cleanup