This results in a 10-20x size reduction per record.
(from ~150kb to ~10kb)

This revision has been pushed live, as I'm in the process of
converting Log records to bz2-compresed CompressedLog records.
I would have waited but we're running seriously low on space
and it seemed like a sane (and reversible) move.

R=rsc
CC=golang-dev
https://golang.org/cl/872048