Lookaside clone support proof of concept

[Augie Fackler]

At the last sprint I talked some about an idea I had to extend (possibly via an extension) the wire protocol to allow some sort of advisory "go fetch from over there first" tricks. I finally got around to doing this today(see [0] for code), and it worked out pretty well. It's a little raw at the moment, but I think docstring on the module explains how to use it well enough. I think this might be most interesting at the moment to Mozilla (as implemented), but it's not hard to envision this letting you direct the client to pull from another (potentially lagged) repository before pulling from the mainline repository to spread out lag over more mirrors.

Thoughts? Anyone got preferences on how I should proceed?

[0]: http://hg.durin42.com/augie/mercurial/lookaside-bundle/file/tip/lookaside.py