added sanity check on repo create/update

This pull adds a check to ensure that the number of packages in the updated repo is equal to the number of packages in the original repo + the number of artifacts being added.

We were seeing createrepo log errors trying to read the synthesized files, then successfully create a repo containing only the added artifacts. This is due to eventual consistency in S3. When downloading the original repo data, we were getting a mismatch between repomd.xml and primary.xml.gz. The plugin synthesizes files based on primary.xml.gz, but if the checksum doesn't match, createrepo ignores primary.xml.gz and attempts to create the repo from scratch. Since the synthesized files are not actual .rpms, createrepo logs an error and ignores the file.