bake: Commonize worker pool code between html and asset baking. The `workerpool` package now defines a generic-ish worker pool. It's similar to the Python framework pool but with a simpler use-case (only one way to queue jobs) and support for workers to send a final "report" to the master process, which we use to get timing information here. The rest of the changes basically remove a whole bunch of duplicated code that's not needed anymore.

#!/bin/sh

CUR_DIR="$( cd "$( dirname "$0" )" && pwd )"
CHEF=${CUR_DIR}/../bin/chef
OUT_DIR=${CUR_DIR}/../piecrust/resources/messages
ROOT_DIR=${CUR_DIR}/messages

$CHEF --root=$ROOT_DIR bake -o $OUT_DIR
rm ${OUT_DIR}/index.html