add options for parallel deltarpm creation

In some Fedora project infrastructure use cases, the createrepo process is dominated
by the time taken to generate deltas.  This change allows delta processing to be done
in parallel but with limits on the total size of the packages being processed at any
given time.
tweak work queue action logging
Patch: allow 'sha' checksum type for modifyrepo

createrepo allows 'sha' as alias for 'sha1', so why not modifyrepo?
Merge branch 'master' of ssh://yum.baseurl.org/srv/projects/createrepo/git/createrepo

* 'master' of ssh://yum.baseurl.org/srv/projects/createrepo/git/createrepo: (2 commits)
  Fix open_checksum and open_size calculation in RepoMetadata.add().
  ...
Merge commit 'imcleod/parallel_deltas_full'

* commit 'bd557727992838efbf5f2b8052cbdeb64dda6704': (2 commits)
  tweak work queue action logging
  add options for parallel deltarpm creation