resolved: handle mDNS timeouts per transaction

mDNS packet timeouts need to be handled per transaction, not per link.
Re-use the n_attempts field for this purpose, as packets timeouts should be
determined by starting at 1 second, and doubling the value on each try.

src/resolve/resolved-dns-transaction.c

@@ -656,8 +656,10 @@ static usec_t transaction_get_resend_timeout(DnsTransaction *t) {
                 assert(t->server);
 
                 return t->server->resend_timeout;
-        case DNS_PROTOCOL_LLMNR:
         case DNS_PROTOCOL_MDNS:
+                assert(t->n_attempts > 0);
+                return (1 << (t->n_attempts - 1)) * USEC_PER_SEC;
+        case DNS_PROTOCOL_LLMNR:
                 return t->scope->resend_timeout;
         default:
                 assert_not_reached("Invalid DNS protocol.");