Adding a new DistroArchSeries involves manual SQL and long transactions

[William Grant]

Adding a new DistroArchSeries to an existing DistroSeries requires a somewhat involved process, documented at <https://wiki.canonical.com/InformationInfrastructure/OSA/LPHowTo/NewDistroArchSeries>. This includes a single long manual INSERT query to clone arch-indep BPPHs to the new architecture, in violation of the new 2.5s transaction limit.

This is very similar to the InitialiseDistroSeries long transaction issue.