Allow split borrow of QueuePair, to make it possible to hold `PostSendGuard` and `PostRecvGuard` at the same time

[dragonJACson]

In the original C API, user could actually post_send and post_recv for a single QP at the same time (on different threads), but current implementation of PostSendGuard and PostRecvGuard won't allow us to do that, we need an implementation that provides split_mut to borrow a part of the QueuePair.

[dragonJACson]

Look into this library, maybe a possible solution: https://github.com/wdanilo/borrow