Always query DNS records over TCP instead of UDP

TXT records can be quite large, with is a problem for DNS challenges,
they don't fit in the 512 buffer especially if there are multiple.
DNS servers can increase the buffer size through EDNS0 extension,
and retry over TCP if that wasn't enough.

For the pebble purposes there isn't much reason to try UDP first,
it is possible to just always query DNS records over TCP, so we do.

Was originally found in
#536 (comment)

Author: kanashimia

va/va.go

@@ -139,6 +139,7 @@ func New(
 	if customResolverAddr != "" {
 		va.log.Printf("Using custom DNS resolver for ACME challenges: %s", customResolverAddr)
 		va.dnsClient = new(dns.Client)
+		va.dnsClient.Net = "tcp"
 	} else {
 		va.log.Print("Using system DNS resolver for ACME challenges")
 	}