Add by-hash support

Add a per-suite boolean to enable by-hash; store the by-hash files in
the db, and record when they stopped being referenced, so that
clean-suites can delete them after the archive's stayofexecution time.

In generate-release, where we have checksums for all the things,
hardlink files to the by-hash dir for each of the suite's configured
hash methods.

Signed-off-by: Julien Cristau <jcristau@debian.org>
---
changes in v2:
- use archive.stayofexecution as delay before removing files from
  by-hash
- don't assume any particular ordering for suite.checksums

changes in v3:
- rebase on latest master, update115 is now update116
- handle missing files in clean_byhash

changes in v4:
- use hardlinks instead of symlinks
- don't initialize `unreferenced` to its default value
- in clean_byhash, remove useless fetchall, and handle ENOENT from
  unlink instead of checking for existence beforehand