Improve load testing tool.

Rewrote/reworked mwalker's `injectMetabooks` script into a more complete
load-testing tool.  This can also be used to quanify failures and find
regressions.

The list of page titles and prefixes in `pages.list` comes from the
Parsoid round-trip testing tool.  It consists of about 10k randomly
selected articles from a number of large wikis.

Change-Id: I8cfe705e366b71606ebd7f5a81c7796d5153145c