- Add function `validateServerAddress()` to `bdns/servers.go` which ensures that
DNS server addresses are TCP/ UDP dial-able per: https://golang.org/src/net/dial.go?#L281
- Add unit test for `validateServerAddress()` in `bdns/servers_test.go`
- Update `cmd/boulder-va/main.go` to handle `bdns.NewStaticProvider()`
potentially returning an error.
- Update unit tests in `bdns/dns_test.go`:
- Handle `bdns.NewStaticProvider()` potentially returning an error
- Add an IPv6 address to `TestRotateServerOnErr`
- Ensure DNS server addresses are validated by `validateServerAddress` whenever:
- `dynamicProvider.update() is called`
- `staticProvider` is constructed
- Construct server addresses using `net.JoinHostPost()` when
`dynamicProvider.Addrs()` is called
Fixes#5463