Adding back-off instead of busy-spinning

Also changed the total number of spinning similar to Cilk
Addressed code review comments.
auto merge of #3820 : cgaebel/servo/rebase-lalehs-patch, r=pcwalton

I addressed all but one of the code review comments, which was a request
for documentation on where a number came from (which I'm not qualified to
answer), and rebased this on to latest master.

xref: #3505 cc: @LalehB @larsbergstrom

r? @pcwalton