uploaders should get emails/notifications for packages in depwait mode

[Fabien Tassin]

Uploaders should get an email after n retries of packages in depwait mode in order to attract their attention.

Today, users get a notification for FTBFS but not for MANUALDEPWAIT.
The first few retries for a package in MANUALDEPWAIT state could be considered normal but after a while, it may indicate an issue, either a really missing dependency that the uploader should address or a broken builder/repository that should be reported to Launchpad admins for investigation.

This notification could be sent after a fixed number of retries (maybe 3 or 5) or after a given period (3h or 6h).
It is also desirable to receive periodic notifications, probably no more than once a day.

Packages should also move from MANUALDEPWAIT to FTBFS after a while, preventing an infinite number of retries (and notifications).