feat: proposal for hybrid session takeover and resumption in emqx 5

[emqplus]

fixes: https://github.com/emqx/eip/issues/59

[zmstone]

Great proposal.

Not sure if QUIC socket owner handover is supported as TCP/TLS socket, if not, we'll need to treat them differently. @qzhuyan ?

[zmstone]

Some discussion points, not necessarily to be added to the EPI, but important to note when developing and testing.

In the current implementation, if we only look at the takeover initialization phase, it consists of 2 steps

1. Sends a nice step-down request to the old
2. If step 1 timeout, force shutdown (by kill the old)

We'll need to consider the scenario of timeouts during local resume (socket handover) too. Some exception flows to cover:

• If the old process receives a new socket while the old socket still active (e.g. no tcp_closed was received), how to proceed? Maybe close the old and accept the new, then we'll need to take care of the state transitions as if the old socket is closed in a normal way.
• If the old process is not responding to socket handover, how to proceed? When timeout happens, good chance the process got stuck somewhere (e.g. calling out to external database for authentication), and the old socket close or error event is maybe in its mailbox already.