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}
还是需要考虑并发操作的问题。
需要保证target_next本身仍在stack中