Use single poll thread

Signed-off-by: Thynson <lanxingcan@gmail.com>
Squash

Signed-off-by: Thynson <lanxingcan@gmail.com>
docs and code clean

Signed-off-by: Thynson <lanxingcan@gmail.com>
redisign spin_poll_target_t

Signed-off-by: Thynson <lanxingcan@gmail.com>