Avoid a needless decode/re-encode step in assigning onion keys

Previously we had decoded the asn.1 to get a public key, and then
discarded the asn.1 so that we had to re-encode the key to store it
in the onion_pkey field of a microdesc_t or routerinfo_t.

Now we can just do a tor_memdup() instead, which should be loads
faster.

src/feature/dirparse/microdesc_parse.c

@@ -177,8 +177,8 @@ microdescs_parse_from_string(const char *s, const char *eos,
                "Relay's onion key had invalid exponent.");
       goto next;
     }
-    router_set_rsa_onion_pkey(tok->key, &md->onion_pkey,
-                              &md->onion_pkey_len);
+    md->onion_pkey = tor_memdup(tok->object_body, tok->object_size);
+    md->onion_pkey_len = tok->object_size;
     crypto_pk_free(tok->key);
 
     if ((tok = find_opt_by_keyword(tokens, K_ONION_KEY_NTOR))) {

src/feature/dirparse/routerparse.c

@@ -588,8 +588,8 @@ router_parse_entry_from_string(const char *s, const char *end,
              "Relay's onion key had invalid exponent.");
     goto err;
   }
-  router_set_rsa_onion_pkey(tok->key, &router->onion_pkey,
-                            &router->onion_pkey_len);
+  router->onion_pkey = tor_memdup(tok->object_body, tok->object_size);
+  router->onion_pkey_len = tok->object_size;
   crypto_pk_free(tok->key);
 
   if ((tok = find_opt_by_keyword(tokens, K_ONION_KEY_NTOR))) {