api request: Add function iperf_server_end (similar to iperf_client_end)

I'm using iperf api in a program but it lacks the ability to programmatically stop the server.
Client api has iperf_client_end but the server one does not have the similar function.

Currently I invoke the method cleanup_server(test) for stopping the server, but by doing this, the application sometimes crashes with the signal 13 (SIGPIPE). The stream could be writing at that time.