This Diff builds upon D21011. A simplistic reflector for
the received CE, ECT1 and ECT0 marked data bytes in the
AccECN TCP option - which is sent on every ACK, in full.
Only when space is needed for SACK, an abbreviated option
is constructed and sent (if possible).
Potential enhancements: Track which counters changed, and
send only changed counters repeatedly (e.g. 3 times at least)