Fix a problem that could disrupt existing tests.

By design, an iperf3 server only runs one test at a time.  New
connections from other clients (during an existing test) are
rejected.  A problem is that the server code that rejects the test
tries (for some reason) to read the cookie from the client, even
though it's going to reject the connection anyway.

A way to break an existing test is:  With a test running, make a TCP
connection to the server's control port (this can easily be done with
a telnet client).  The server will hang in a blocking read call trying
to read the cookie from a non-existent client, while the test is
essentially frozen.

The fix is to remove the attempted read of the cookie.

Fixes #202.