Program the upstream IPv6 map in BpfCoordinator.

- Add methods to start and stop IPv6 forwarding upstream
- Populate the upstream IPv6 map when the first rule for any
  upstream/downstream pair is created.
- Clear the upstream IPv6 map when the last rule for any
  upstream/downstream pair is deleted.

Test: Added coverage to IpServerTest and BpfCoordinatorTest
Change-Id: Ib041081e95f5f449489ab63138de034222ffac8f