stat: added per-backend command (requests/bytes and more) statistics
session: fixed zero-length write - we must return error and do not attempt to write data
session: long lines cleanup
stat: added success/error requests in per-backend command stats
stat: added success/error requests in per-backend command stats
stat: fixed input stat name for success command counter
stat: added rps/bps counters, use pointers to command stat structs to allow in-place update