https://gerrit.fd.io/r/#/c/8551/ decoupled the global variable, namely tm->iovecs from TX and RX. However, to support multi-threads, we have to eliminate the use of this global variable with per thread variable. I notice that rx_buffers must also be per thread variable. So, we introduce per thread struct to contain rx_buffers and iovecs. Each thread will find the per thread struct with thread_index.