Add a `MessageContext::DNSResolution` to protect against probing

Add a `MessageContext::DNSResolution` to protect against probing

When we make a DNSSEC query with a reply path, we don't want to
allow the DNS resolver to attempt to respond to various nodes to
try to detect (through timining or other analysis) whether we were
the one who made the query. Thus, we need to include a nonce in the
context in our reply path, which we set up here by creating a new
context type for DNS resolutions.