Add support for 'atomic' purge-and-resync of repositories

We need to be able to resync repositories from scratch as needed. This is a useful dev tool, but also a useful tool for a website like d.o, where occasional problems crop up with a repository's data and a normal resync doesn't solve the problem (since syncing from scratch is almost always correct).

Fundamentally, this operation is just $repo->purgeData(); $repo->fetchLogs(). However, since other systems expect to be able to build on vcapi & rely on its serial ids, those two steps need to be wrapped in an atomic wrapper that fires pre- and post- hooks so that they can do what they need to preserve & update their data. Example from d.o - versioncontrol_project has to respond and rebuild associations between labels and releases - basically, reupdate the {versioncontrol_release_labels} table with the new label ids for resynced labels.