message: fuzz: improve fuzz performance for accesslog and route
  This PR includes some performance improvements to route_fuzz_test.
  gMock objects that are unmodified are now static (see recommendation by libFuzzer: https://llvm.org/docs/LibFuzzer.html#startup-initialization)
  Exec/sec on my local machine with libFuzzer:
  route_fuzz_test: 17 exec/sec -> 850 exec/sec
  Risk level: Low
diff: @@ -36,7 +36,6 @@ cleanRouteConfig(envoy::api::v2::RouteConfiguration route_config) {
  // headers.
  envoy::api::v2::RouteConfiguration clean_config =
  replaceInvalidHeaders<envoy::api::v2::RouteConfiguration>(route_config);
  - // Replace invalid characters from
  auto internal_only_headers = clean_config.mutable_internal_only_headers();
  std::for_each(internal_only_headers->begin(), internal_only_headers->end(),
  [](std::string& n) { n = Fuzz::replaceInvalidCharacters(n); });
  @@ -69,9 +68,9 @@ cleanRouteConfig(envoy::api::v2::RouteConfiguration route_config) {
  // TODO(htuch): figure out how to generate via a genrule from config_impl_test the full corpus.
  DEFINE_PROTO_FUZZER(const test::common::router::RouteTestCase& input) {
  + static NiceMock<Envoy::StreamInfo::MockStreamInfo> stream_info;
  + static NiceMock<Server::Configuration::MockFactoryContext> factory_context;
  try {
  - NiceMock<Envoy::StreamInfo::MockStreamInfo> stream_info;
  - NiceMock<Server::Configuration::MockFactoryContext> factory_context;
  TestUtility::validate(input.config());
  ConfigImpl config(cleanRouteConfig(input.config()), factory_context, true);
  Http::TestHeaderMapImpl headers = Fuzz::fromHeaders(input.headers());
  