rename conn->timestamp_lastwritten to conn->timestamp_lastwritable
"lastwritten" sure sounds like we actually flushed something or we actually added a new thing to write.
time_t timestamp_lastwritten; /**< When was the last time libevent said we * could write? */
It seems that we changed our definition of this word somewhere along the line, but we left the old word in place. How confusing.
We might want to change lastread to lastreadable too.