renove data sockets closing from server_timer_proc as it caused threa…

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any): none

• Brief description of code changes (suitable for use as a commit message):

While evaluation Issue #1986 and simulating disconnect between the client and server, the server crashed after the call to server_timer_proc(). Using gdb I found that the problem was that the sp->test address in the data thread was garbled. I believe that this was because of race condition between the main and data threads:

1. server_timer_proc() closed the data stream and freed the sp buffers.
2. The sp buffer was used for something else, garbling the test address` (if this step happens later, depending on the threads race conditions, the crash will not happen).
3. Since the data stream was closed, the data thread read()/recv() returned 0 - end of file.
4. iperf_tcp_recv() in the data thread performs the if (sp->test->state == TEST_RUNNING) test that cause the server crash because the sp->test is garbled.

The suggested fix is that server_timer_proc() will only close the control socket, but not the data streams. This causes the select() to fail and then the threads/sockets are closed in the usual way after failure.

Another approach was that server_timer_proc() will close the threads before closing the streams, but I think just closing the control channel is better.

Also, I noted that server_timer_proc() does not issue any error message. I didn't add such message as I am not sure if this is not intentional.