UDD up-to-date

[Martin Pool]

We're in the middle of a fairly epic series of roll-outs to the
Launchpad buildds.  Done today, both on staging and production, is an
update to bzr 2.4, which should let large trees be assembled in recipe
builds without running out of memory.

Coming up soon are upgrades to the buildd client mechanism to make it
more supportable, and to allow building non-native packages
(https://bugs.launchpad.net/launchpad/+bug/885497) and some other
things.

Thanks to Lamont, Jelmer, wgrant, and everyone else involved.

m