dns watcher ping assumes dns server resolves public addresses

The dns watcher's ping method declares the watcher dead if it can't resolve airbnb.com. This doesn't work if you are in a private network (e.g. AWS VPC) and are using a DNS server that only resolves private domains.

Potential alternatives:
- allow the domain for the ping to be configurable
- don't care if the domain is not found (the fact that it got a proper response from the dns server is good enough)

Inspect the DNS watcher's ping method and reproduce its behavior with a DNS server that resolves only private domains. Decide which proposed behavior the project should use, then verify that a valid response from the DNS server no longer incorrectly marks the watcher dead.

Written by the indexing model from the issue text.