Smart server fetching update: 7x faster

[Andrew Bennetts]

Just a quick note to say that the branch that adds a
“Repository.fetch_revisions” request to the smart server is nearly 7 times
faster at fetching 10 mainline revisions (132 total revisions) from London ->
Sydney.  That's 1.5MB of revision data.  Roundtrip time on this connection is
~350 ms.  Here's the numbers:

“bzr pull” into a branch that's 10 revisions behind the remote copy, using my
repo-refactor branch at both ends over bzr+ssh:

real    1m2.774s
user    0m6.668s
sys     0m0.592s

“bzr pull” into a branch that's 10 revisions behind the remote copy, using
bzr.dev over bzr+ssh:

real    7m5.129s
user    0m9.273s
sys     0m0.416s

It's still downloading the revisions.kndx, which is more than twice as large as
the revision data being transferred in the fetch_revisions request, so I expect
we can do even better.  Also note that it takes 8s to establish the SSH
connection, and that's included in these times.

There's also a few seconds wasted re-opening the remote branch after the
revisions are transferred (with both versions of bzr).  I haven't tracked that
down yet, but that will also improve these numbers.

-Andrew.