Only delete un-referenced checksums

When updating a cookbook version, only checksums that are no longer
associated with any cookbook version (not just the one being updated)
should be deleted from S3.
Rename checksum deletion functions

Renamed the following:

  delete_cookbook_checksums -> unlink_checksums_from_cbv
  delete_cookbook_version_checksums -> unlink_all_checksums_from_cbv
  delete_checksums -> delete_orphaned_checksums

This should remove any ambiguity over what these functions do.