Smart protocol status update

[Andrew Bennetts]

This is just a brief update about the state of the HPSS code.  There's been a
few things going on lately.

First, a nice result: with all the patches I've posted to the list (including
the still quite rough), I can push a handful of new revisions onto an existing
bzr branch from Sydney to London in 28s, including 8s SSH connection setup, and
including autopacking.  Some of that work isn't merged yet, but it's close.

There's lots of room for improvement on pushing still, but that's pretty good.
The next obvious areas of improvement are pack file management (e.g. avoiding
roundtrips for renaming index/pack files into place and for updating
pack-names), and not using VFS for creating new branches and repositories (which
causes a fair few round trips as it builds directories and files one-by-one, and
acquires, releases and reacquires locks files).

The code in bzr.dev, which includes protocol v3, seems pretty solid.  There
hasn't been an unusual surge in HPSS bugs reported against bzr.dev since it
landed, so I'm quite happy about the stability of it.

Robert's versionedfiles work currently undoes some of the streaming pull work in
HPSS.  We need to measure the impact of this and figure out what to do about it.
It's probably not too hard to implement new pull streaming in terms of
{get,insert}_record_stream, but with packs it might not big performance hit.

-Andrew.