reftable: implement new backend

@chriscool was implementing the reftable format introduced first in jGit. Which would dramatically improve disk space used and increase ref scan and lookup times.

This work was mostly sponsored by another company, but when there was residual capacity within GitLab, GitLab could dedicate those resources.

Since November 2019 though, Han-Wen Nienhuys from Google (which maintains jGit and implemented reftable in jGit) has been working a lot on a separate implementation that would be common to Git and libgit2.