Avoid socket re-registeration

[ustulation]

More details here in the review comment.

Summarising again, the uniform idiom should be to just read in a loop. If you collect many msgs and only one is meant to be handled by the current state, then pass the remainder over to the next state. Similarly never re-register to write (or avoid it unless impossible to do without it). Just write instead. Start with registering once for read and write with edge notification and work your way from there.

This avoids forgetting the correct re-registerations and undoing previous ones and doing new ones etc. Doing all that is utterly nuanced, error prone, causes breakages easily during code changes and makes it difficult to reason about properly in reviews.