Start the iptables and ip6tables processes in parallel.

This saves ~30ms on boot.

There should be no races between the threads that start the
and the main thread because pthread_join is documented to
synchronize memory with respect to other threads.

Bug: 28362720
Test: bullhead builds, boots
Test: netd_{unit,integration}_test pass
Change-Id: I24d83b880bd011bc801178f57f46d90f36621f9f