BUG/MINOR: time: make sure only one thread sets global_now at boot

BUG/MINOR: time: make sure only one thread sets global_now at boot

All threads call tv_update_date(-1) at boot to set their own local time
offset. While doing so they also overwrite global_now, which is not that
much of a problem except that it's not done using an atomic write and
that it will be overwritten by every there in parallel. We only need the
first thread to set it anyway, so let's simply set it if not set and do
it using a CAS. This should fix GH issue #111.

This may be backported to 1.9.