Configure IPv6 DNS server using router advertisements.

[jglogan]

• Closes [Request]: IPv6: allow DNS configuration update after container.create(). #466.
• Adds a DNSMonitor that listens for router advertisements and updates the resolv.conf with the IPv6 address of the DNS server found in the RDNSS option, if present.
• The monitor runs if any of the configurations registered via configureDns have enableRDNSSMonitor set to true, and stops if all are false.
• The monitor removes the IPv6 server from resolv.conf when either
  • the monitor stops.
  • the current RA expires.
• Adds updateDNS() to LinuxContainer and LinuxPod; we use these in the integration tests to ensure the server removal logic works.
• The configureDns function now calls DNSMonitor.update() to write the initial configuration.
• The monitor does not block container workload startup. IPv6-only containers will need to tolerate the initial lack of a DNS server.

[jglogan]

Example

container run --name foo -it --rm alpine cat /etc/resolv.conf
Warning! Running debug build. Performance may be degraded.
nameserver 192.168.64.1
nameserver fe80::603e:5fff:fe94:4e65
domain test

Performance impact of blocking implementation

Baseline (no IPv6 DNS configuration):

container run --name foo -it --rm alpine date  0.09s user 0.03s system 11% cpu 0.992 total
container run --name foo -it --rm alpine date  0.08s user 0.03s system 12% cpu 0.871 total
container run --name foo -it --rm alpine date  0.09s user 0.04s system 12% cpu 0.949 total
container run --name foo -it --rm alpine date  0.09s user 0.03s system 13% cpu 0.899 total
container run --name foo -it --rm alpine date  0.08s user 0.03s system 12% cpu 0.904 total

With IPv6 DNS initialization in configureDns, first container (vmnet network not started):

container run --name foo -it --rm alpine date  0.10s user 0.07s system 3% cpu 4.432 total
container run --name foo -it --rm alpine date  0.10s user 0.07s system 3% cpu 4.473 total
container run --name foo -it --rm alpine date  0.10s user 0.07s system 3% cpu 4.338 total
container run --name foo -it --rm alpine date  0.10s user 0.07s system 3% cpu 4.444 total
container run --name foo -it --rm alpine date  0.10s user 0.06s system 4% cpu 4.012 total

With IPv6 DNS, second container (vmnet network already started):

container run --name foo -it --rm alpine date  0.09s user 0.05s system 4% cpu 2.929 total
container run --name foo -it --rm alpine date  0.09s user 0.05s system 4% cpu 2.952 total
container run --name foo -it --rm alpine date  0.10s user 0.06s system 3% cpu 3.990 total
container run --name foo -it --rm alpine date  0.09s user 0.06s system 5% cpu 2.896 total
container run --name foo -it --rm alpine date  0.09s user 0.05s system 5% cpu 2.790 total