message: Fix an assertion failure about "!owners_ptr->in_use()" in the tests
  The retry logic for picking a random port didn't actually work as
  Server::start() doesn't provide the strong exception guarantee, and calling it
  again after it throws isn't supported. Fortunately it's not needed at all as
  Server now supports picking its own dynamic port.
diff: @@ -156,6 +156,7 @@ SyncServer::SyncServer(StartImmediately start_immediately, std::string local_dir
  config.history_ttl = 1s;
  config.history_compaction_interval = 1s;
  config.state_realm_dir = util::make_temp_dir();
  + config.listen_address = "127.0.0.1";
  return config;
  })())
  @@ -166,20 +167,8 @@ SyncServer::SyncServer(StartImmediately start_immediately, std::string local_dir
  SyncManager::shared().set_log_level(util::Logger::Level::off);
  #endif
  - uint64_t port;
  - while (true) {
  - // Try to pick a random available port, or loop forever if other
  - // problems occur because there's no specific error for "port in use"
  - try {
  - port = fastrand(65536 - 1000) + 1000;
  - m_server.start("127.0.0.1", util::to_string(port));
  - break;
  - }
  - catch (std::runtime_error const&) {
  - continue;
  - }
  - }
  - m_url = util::format("realm://127.0.0.1:%1", port);
  + m_server.start();
  + m_url = util::format("realm://127.0.0.1:%1", m_server.listen_endpoint().port());
  if (start_immediately)
  start();
  }
  