Add a finishImport() method to FeedsSource to bundle tasks that need to be done at the end of an import

Right now the methods FeedsSource::import() and FeedsSource::pushImport() both perform the same tasks when the import task is done, with the exception that FeedsSource::pushImport() doesn't invoke the rules event 'feeds_after_import' (which I think it should do).

Let's combine these tasks that need to be done at the end of an import into one method: FeedsSource::finishImport().

This is inspired by the D8 version of Feeds where the \Drupal\feeds\Entity\Feed class (equivalent of FeedsSource in D7) also has a method with the same name.

Patch will follow.