make rake canvas:compile_assets a TON faster

...by parallelizing everything I could

fixes: CNVS-12362

`time bundle exec rake canvas:compile_assets`
(on my I7 2.3GHz macbook pro)
before: "322.56s user 26.62s system 173% cpu 3:21.05 total"
after: "425.91s user 30.71s system 413% cpu 1:50.35 total"

If you ever find yourself needing to run everything all
sequentially (and not in multiple threads), you can
set the environment variable CANVAS_BUILD_CONCURRENCY=1
by default it will use all the cores your system has

You're welcome, anyone that has ever had to build canvas
or wait for jenkins to run :)

Test Plan:
* run "time bundle exec rake canvas:compile_assets"
* checkout this code
* run it again
* verify that it is faster, and everything works

Change-Id: Ib01506bc9638e86f4a329284458706279ef751ab
Reviewed-on: https://gerrit.instructure.com/33127
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>