godoc: parallelize corpus init
godoc: parallelize corpus init

Massive win for high-latency network filesystems.
Also benefits the local disk/ssd case too, though.

R=golang-dev, bgarcia
CC=golang-dev
https://codereview.appspot.com/18650043

d0f/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,skyportsystems/go.tools,d0f/go-zh.tools,d0f/go-zh.tools