Podcast feed update status fixes

• podcast: make feed status 'updating' as transient status

      This is primarily needed for handling feed update cancellations during
      rhythmbox close, as there is no definite timeline from plparser on how
      quickly it will detect the cancellation and invoke the user
      callback. So, rather than waiting / blocking for plparser callback
      during rhythmbox close, we make the feed 'updating' status as
      transient.

• podcast: update feed status on feed update cancellation