add code for no thread for run request

QPS 下降验证， 单线程以下降了 33%， 调查原因

$ out/opt/net/rpc/async_benchmark  --event_poll_threads=4
E1018 05:29:00.856679 20145 async_benchmark.cc:30] start to send
E1018 05:29:03.205678 20145 async_benchmark.cc:38] all completed
E1018 05:29:03.205734 20145 async_benchmark.cc:39] 800000 completed in 2348 ms
 QPS: 340000

$ out/opt/net/rpc/async_benchmark
E1018 05:28:41.126405 20108 async_benchmark.cc:30] start to send
E1018 05:28:44.925856 20108 async_benchmark.cc:38] all completed
E1018 05:28:44.925897 20108 async_benchmark.cc:39] 800000 completed in 3799 ms
 QPS: 210000
Merge remote branch 'origin/bugfix_rpc-0.1.6.rc' into bugfix_rpc-0.1.6.rc