mutable publish: add timing charts to measure RTT

[warner]

We should examine the mutable-file publish process to try and get small files to do read-modify-write in two RTT. Large files should be three RTT (one to update the servermap, a second to read the data, a third to write the data).

The first step for this will be better status/visualization tools. Servermap update has a little diagram (using the Google Chart API) of how the queries are spaced out. I'd like to:

• make these charts for all mutable file operations
• for retrieve, show both the mapupdate queries and the retrieve queries on the same chart
• for publish, show mapupdate and publish on the same chart
• (maybe) for modify, show everything on the same chart
• switch to PIL to get better diagrams and show overlapping queries

I think we may be at 3RTT already, but I'm not sure, and better tools would help me be sure.

[warner]

I think we're at two RTT for small files, but I still want better diagrams, so I'm changing the title of this ticket.

[zooko]

It's really bothering me that mutable file upload and download behavior is so finicky, buggy, inefficient, hard to understand, different from immutable file upload and download behavior, etc. So I'm putting a bunch of tickets into the "1.8" Milestone. I am not, however, at this time, volunteering to work on these tickets, so it might be a mistake to put them into the 1.8 Milestone, but I really hope that someone else will volunteer or that I will decide to do it myself. :-)