Requests for comment/Replacing extension distributor

ExtensionDistributor is in need of some major rework.

Problem

 * 1) The code is overly complex (shelling out to git & tar is error-prone)
 * 2) The current system requires NFS on the apaches to checkout and tar the repositories
 * 3) The current system was built for our SVN-based branching model, and doesn't suit users well
 * 4) Ownership and permissions mismatches have meant that the extension is often broken in practice for our users.

Proposed solution

 * Rewrite the code to fetch tars/zips from Gitweb/Gitblit (configurable), since it already handles the "package and tar" logic for us. This solves problems 1 and 2.
 * Put something in front of Gitweb/Gitblit to lessen the load (suggestion: Varnish). We don't actually need to store the archives, long-term caching will suffice.
 * By default, do snapshots of extension master as of a given branch point for core (this was more or less what we accomplished in SVN). This solves problem 3.