MINOR: time: change the global timeval and the the global tick at once

MINOR: time: change the global timeval and the the global tick at once

Instead of using two CAS loops, better compute the two units
simultaneously and update them at once. There is no guarantee that
the update will be synchronous, but we don't care, what matters is
that both are monotonically updated and that global_now_ms always
follows the last known value of global_now.