The Release files in dists/${suite} are now symlinks to zzz-dists.

ce54b4f1 missed changing the variable
name in one place.

Use OpenSSH's `ControlPersist` option instead of implementing an
approximation ourselves.  This avoids an infinite sleep in case the
`ssh` call fails and the socket never appears.

Rather than keying on the byhash boolean, make Release always a symlink
to zzz-dists.
Signed-off-by: NJulien Cristau <jcristau@debian.org>

The byhash boolean now controls:
- whether Release lives in zzz-dists (should we change this too?)
- whether Acquire-by-hash is advertised in Release

This should help speed up the second stage of mirroring.
Signed-off-by: NJulien Cristau <jcristau@debian.org>

This silences a warning from newer SQLAlchemy releases:

    SAWarning: Textual SQL expression '[...]' should be explicitly
    declared as text('[...]')

Making the same error when adding the file locally in /usr/share/dpkg
for testing doesn't help with noticing this.

That doesn't store original name/timestamp, with the latter meaning a
change even if there wasn't one in the input. Ought to help systems like
snapshot.

While not really relevant, this change stops `check-override` from
trying to sync the source package's override with every run (as the
priority continued to differ).