A practical improvement need not be over-complicated; nor does it need to be overly general.   Database transactions, and for that matter the TCP/IP protocol itself, are more than sufficient to deal with “out-of-order packets.”   What we need here is a pragmatic improvement upon the sort of situation that the OP has described ... a session-store that is more cognizant of the practical needs of AJAX transactions ... and, a wheel that has no need of being re-invented.   That Is All.

“We can rebuild him.   We have the technology ...”