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I tried to implement that approach but unfortunatelly I was not able to make it run in my asynchronous state machine.
It looks somehow like this now:
The Map is being locked everytime it is being used.
Every now and then something wants to send data -> map is being locked -> send.
During that send we have the case that it somehow hangs -> deadline timer -> check_destroy() -> onclose()-handler.
The onclose wants to lock the map as it wants to remove the connection but this is not possible as the map is still being locked by the send (deadlock).
Ok so now I outsource the onclose-task in a separate thread that waits for the send to finish and then locks the map to remove the connection.
Now "delete this" is being called as a final step of check_destroy() in the send task.
send-task returns and in an ideal world my outsourced thread will now recognize that it can lock the map to remove the deleted connection.
However, my asynchronous state machine decides to send another data and is faster than the outsourced thread.
Send() locks the map again and runs into a segfault as the connection had been deleted by itself.

Don't get me wrong. I am fine with your idea to have a very simple library and if you want to have check_destroy() being called in such a case then go for it, however, it did not work for me.