outbuf_flushlen seems to serve no purpose
The "outbuf_flushlen" field in connection_t seems to not actually do anything that's distinct from buf_datalen(conn->outbuf)... except possibly, to get out of sync with it under rare conditions? (eg legacy/trac#32472 (moved)).
Flushlen once existed to implement rate limiting: it was introduced in 117cbeea before we even had round robining. Later there was some logic involved with f5ebf4c7 to try to flush full TLS records. And controller connections used ab838bdd to force early flushing there... but right now, we don't flush according to the same logic that we used to flush, and I think outbuf_flushlen is now obsolete.
I think there's a case to be made for removing this field in 0.4.3, but I'd like to be cautious.
Adding arma to cc in case he can remember what outbuf_flushlen is for.