prevent lookup of conn_id 0x00010000 or 0xFFFFFFFF when adding or subtracting 1 after casting uint16 to uint32
do not bother with printf if the level is not enabled; allow callers to avoid gathering expensive printf arguments by querying whether a level is enabled before calling log
allow utp_contexts running in different threads to utp_writev at the same time without overwriting each other's utp_iovecs
fix off-by-one error when appending to rst_info