Gets the total number of messages written to the write buffer
on a connection. This number usually is the same as the
total number of messages actually sent on the connection, but
sometimes the retrieved number is larger than the number
of messages sent when the write buffer is not frequently
flushed.
Note: The result is truncated to 31 bits, and may contain an
incorrect sign bit on some older clients. New clients should use
TrafficMsgsSent8.