Now that we require rayon 0.7, we can register start and stop handlers
that take care of dealing with the Gecko profiler for rayon threads in
the style thread pool.
I wanted to add an environmental variable to disable the style sharing
cache for gecko, but the current pattern involves lazy_static!, which
involves an atomic operation on lookup, which is a bit hot to do each
time we try to share styles. This makes that work happen once per
process.
