[P2P] Refactor P2P code

[zhongjiechen]

Refactor P2P code to make it much easier to maintain.
Refer to the latest PR #571

• EFA
• Normal RDMA NICs

[YangZhou1997]

One transport we also want to support is TCP, though it has a much different semantic with RDMA. Our interface should be compatible with it as well

[YangZhou1997]

Update: I am adding TCP support following efa style for now.