Bug#818498: migrate from debmirror to archvsync

[Stefano Zacchiroli <zack@debian.org>]

Package: qa.debian.org
Severity: important
User: qa.debian.org@packages.debian.org
Usertags: debsources

debmirror shortcomings have been bugging debsources for quite a while lately.
Let's migrate away from it to archvsync [1], as that will put us in a more
"normal" situation w.r.t. how actual Debian mirrors are maintained around the
world.

[1]: https://ftp-master.debian.org/git/archvsync.git

I'm marking this bug as important because, after the recent archive changes,
debmirror isn't properly mirroring experimental (due to its having been
switched to Sources.xz).

Cheers.

-- System Information:
Debian Release: stretch/sid
  APT prefers testing
  APT policy: (500, 'testing'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.3.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=it_IT.utf8, LC_CTYPE=it_IT.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)