Upstream filters and filter state interaction.

Title: Crash on retry when writing read-only data from upstream filter.

Description: Note: this has already been run by the envoy security team, and it's been decided that we can do this in the open.

Adding read-only data to downstream filter state in an upstream filter will crash Envoy when a retry happens. For example, the following test will crash envoy:

TEST_P(MultiplexedIntegrationTest, RetryWithFilterState) {
  test::integration::filters::HeaderToFilterStateFilterConfig config;
  config.set_header_name("x-uh-oh-header");
  config.set_state_name("uh-oh-state");
  config.set_read_only(true);
  envoy::extensions::filters::network::http_connection_manager::v3::HttpFilter filter;
  filter.set_name("header-to-filter-state");
  filter.mutable_typed_config()->PackFrom(config);
  config_helper_.prependFilter(MessageUtil::getYamlStringFromMessage(filter), false);
  // also add the x-uh-oh-header header.
  testRetry();
}

You might think that choosing to add the header in upstream filter state (downstream_callbacks_->streamInfo().upstreamInfo()->upstreamStreamInfo()) may be a better option, but it is not. This is because the upstream stream info is reset halfway through UpstreamRequest::onPoolReady(), which may be called after the filter chain has already decoded the headers.

Call Stack:

Unhandled exception:
    @     0x562c24625bb2  Envoy::StackTrace::terminateHandler()
    @     0x562c25539886  std::__terminate()
    @     0x562c25538f7b  __cxxabiv1::failed_throw()
    @     0x562c25538ef4  __cxa_throw
    @     0x562c2475f71f  Envoy::StreamInfo::FilterStateImpl::setData()
    @     0x562c22d0d0e0  Envoy::HeaderToFilterStateFilter::decodeHeaders()
    @     0x562c24273aa5  Envoy::Http::FilterManager::decodeHeaders()
    @     0x562c2424869d  Envoy::Router::Filter::doRetry()
    @     0x562c24bee4f7  event_process_active_single_queue
    @     0x562c24beab4c  event_base_loop
    @     0x562c22433721  Envoy::Server::WorkerImpl::threadRoutine()
    @     0x562c24bdd163  Envoy::Thread::ThreadImplPosix::ThreadImplPosix()::{lambda()#1}::__invoke()
    @     0x7f65400717d9  start_thread
    @     0x7f653ffe469f  clone
libc++abi: terminating due to uncaught exception of type Envoy::EnvoyException: FilterState::setData<T> called twice on same ReadOnly state.