Shared keepalive v3

Changes in V3:
- Adjust some coding style.
- Remvoe zero array in keepalive_shared_srv_t.
- Remove assert().
- Fix ngx_channel_handler().
- Ignore connections when its owner process is dead.
- Delete old shared memory when relaod Tengine.
- Update Copyright in ngx_lfstack.{h,c}
如果已发送了请求，但是对方没有正常响应的情况下，这个连接会不可感知，直到keepalive timeout发生。另外在keepalive timeout事件中，内存不会回收进info->idle_list