Add VFS for local disk

Part of #439 (closed)

This makes all interactions with the fileystem in internal/serving/disk pass through the virtual filesystem interface. This includes symlink resolution with internal/serving/disk/symlink.

We also add Prometheus counters for the 3 basic filesystem operations (Lstat, Readlink, Open).

This is based on !315 (closed). The purpose of the virtual filesystem interface is to allow us to serve pages either from local disk, as we do now, or from object storage.