Preserve caller context across DNS lookups (#496)

Preserve caller context across DNS lookups (#496)

For now, the context is restored when communicating with _individual_
callers waiting for their DNS lookup results. Eventually, we might also
support establishing a multi-caller context during DNS answer parsing,
before individual callers are notified. That feature would most likely
require making idns_query refcount-based (a serious change). Or we could
realize that maintaining a very basic query ID-based context is enough.