Keep old updates in -updates while new one is being phased

Bug 1954534 is an example of what happens when you already had a version from -updates installed, but then updates received a new version that's phasing, and you want to install a new binary from the source package: The installation fails, as the release pocket is unfortunately the candidate.

I'd like to suggest that we retain an older version regardless of phasing. I've helped several people roll back regressions by downloading binary packages off launchpad by hand, and that's not great fun:

- there's no gpg chain of trust
- constructing urls to wget for N packages takes effort

Expiring packages two weeks after they are superseded would solve more issues than just phasing updates getting out of sync for whatever reason.

Thanks

That's probably also easier to implement I'd guess, seems like a good idea to me.

Oh Seth if we phase longer than two weeks, we still also want to keep the old version around

That's actually a much more complicated change when you work through all the ramifications (especially if it were to involve keeping the older version in the main Packages file rather than an older by-hash version - that would require delaying the status change to `SUPERSEDED`, and it's not at all obvious to me how that could be arranged given the design of the dominator and the publisher). I'd prefer not to derail this bug onto that.

The dominator change needed for the original request in this bug still involves some hard thinking, but I at least understand roughly how it would work: I think we'd have to consider publications with a non-None `phased_update_percentage` as being ineligible to dominate anything but another publication with the same version.