Implement v5 support with current `EventLoop`

[de-sh]

In the v5 module, we have significantly modified the architecture to solve a certain few problems, we believe this process has bred a new set of problems. Now we seek to undo these problems by re-implementing v5 support with the same old EventLoop::poll way of doing things. This issue will be used to track progress and current v5 implementation will be shelved/moved into a separate branch for future reference when we pursue the restructure wholeheartedly.

• [x] Create restructure branch, with current state of v5, freeze it for future reference
• [ ] Rewrite v5 module with new v5 structs from rumqttd(without properties/topic aliasing)
• [ ] Test and verify v5 support as implemented