* fixed typo in LeafServer::handle__status(): xtavg keys was actually reporting qtavg
* EMA tweaks in DDCTimeAvg
- redefined DDCTimeAvg::TimeT as DDCTimeHiRes: use high-resolution clock if available for EMA
- handle counter overflow in DDCTimeAvg::Append()
- use current timestamp TimeT::Now() for DDCTimeAvg::operator+=(): avoid out-of-order insertions