Fix sync issue with insert_async, better default unordered_set size.
Fix sync issue with insert_async, better default unordered_set size.

buaasun/grappa,buaasun/grappa,uwsampa/grappa,alexfrolov/grappa,buaasun/grappa,alexfrolov/grappa,uwsampa/grappa,uwsampa/grappa,alexfrolov/grappa,buaasun/grappa,alexfrolov/grappa,uwsampa/grappa,buaasun/grappa,alexfrolov/grappa,uwsampa/grappa,alexfrolov/grappa,uwsampa/grappa,alexfrolov/grappa,buaasun/grappa,uwsampa/grappa,buaasun/grappa