use transactions in 'refs_create_symref()'

The patch series takes over from the existing patch series, wherein we introduced symref-* commands to git-update-ref. Since there was a ton of discussions on the UX of the patch series and its application, I thought it would be best to shorten the series and split it into multiple smaller series.

This series adds transactional support for symrefs in the reference db. Then we switch refs_create_symref() to start using transactions for symref updates. This allows us to deprecate the create_symref code in the ref_storage_be interface and remove all associated code which is no longer used.

We also disable the reference transaction hook in this series and add a 'TODO' noting that the hook support will be added in the future, this will be done when we add symref commands to 'git-update-ref' in the next series.

The split was primarily done so we can merge the non-user facing parts of the previous series. While pertaining the user facing components into another set of patches wherein deeper discussion on the UX can be held without worrying about the internal implementation.