MINOR: resolvers: renames some resolvers specific types to not use dns prefix

MINOR: resolvers: renames some resolvers specific types to not use dns prefix

This patch applies those changes on names:

-struct dns_resolution {
+struct resolv_resolution {

-struct dns_requester {
+struct resolv_requester {

-struct dns_srvrq {
+struct resolv_srvrq {

@@ -185,12 +185,12 @@ struct stream {

        struct {
-               struct dns_requester *dns_requester;
+               struct resolv_requester *requester;
...
-       } dns_ctx;
+       } resolv_ctx;