'meteor admin make-bootstrap-tarballs'

other changes:

- defaultReleaseVersion is not synced using 'collections' any more,
  since it is a singleton and has different merge logic
- publish-release --fromCheckout renamed to --from-checkout
- $METEOR_SAVE_TMPDIRS env var to not delete files.mkdtemp stuff
- release.latestDownloaded() now comes from tropohouse

we recognized that the cross-linking stuff in
Tropohouse.maybeDownloadPackageForArchitectures doesn't quite work so we
changed it to not quite work in a different way (which allows us to
leave downloaded-builds out of the bootstrap tarball and decrease its
size by 50%).  will fix later.