add send queue size limit to resolve message backlog.

[LeoYang-Chuese]

• add a function to get the size of the send queue.
• add the maximum size parameter of the send queue, limit the length of the send queue, and discard the latest message beyond the queue limit.

[qicosmos]

还可以增加达到上限的一个回调，回调给用户，让用户有机会做反压的处理。