Investigate OSX client perf

[larsbergstrom]

Below are some notes on raw OSX builder performance, comparing our mac minis with a mac pro. HUGE improvement on test-css and some improvement on test-wpt/compile.

servo-mac3 (mini), mac-rel-css
compile (./mach build --release) 16min, 30secs
test-css (./mach test-css --release --processes 4 --log-raw test-css.log) 29min, 29secs

servo-mac2 (mini), mac-rel-wpt
compile (./mach build --release) 15mins, 15sec
test-wpt (./mach test-wpt --release --processes 4 --log-raw test-wpt.log) 17 mins, 27sec
test-css (./mach test-css --release --processes 4 --log-raw test-css.log) 25min, 46secs

servo-macpro1
compile from full clean (./mach build --release) 16min, 55sec
compile from target-clean (./mach build --release) 12min, 57sec
test-css (./mach test-css --release --processes 4 --log-raw test-css.log) 15mins, 23sec
^ stayed around 15-20% idle the whole time. WindowServer 33% cpu, Terminal 35% cpu. autohide dock took 10% off cpu usage of WindowServer.
test-css (./mach test-css --release --processes 6 --log-raw test-css.log) 14mins, 48sec
test-css (./mach test-css --release --processes 8 --log-raw test-css.log) 14mins, 43sec
test-wpt (./mach test-wpt --release --processes 4 --log-raw test-wpt.log) 15min 57sec

cc @metajack @edunham

[larsbergstrom]

Filed servo/servo#9513 for a pretty obvious big perf win for our OSX testing.

[larsbergstrom]

Given that the macpro costs 2.5x, we should stick with minis and investigate sccache to speed up builds.