[RFC] Interfaces for `modify_qp` / connection setup

[dragonJACson]

A lot of wrappers of rdma-core choose to use builder pattern to create qp and modify qp. We would like to choose export modify_qp directly to user, as we are just a safe wrapper instead of some higher level abstract. But the interfaces are TBD.

• https://github.com/IcicleF/rrddmma/blob/a0050752b8df60ddfca620c27ee6fdaa37b59117/examples/loopback.rs#L10
• https://fasterthanli.me/series/making-our-own-ping/part-6
• https://github.com/jonhoo/rust-ibverbs/blob/55be7d2687a04df8bfa13c27456e5a39da25b32f/ibverbs/src/lib.rs#L1092