2 primary changes made related to iperf command. to fix 2 issues.
when user on mininet console issues a command : iperf h1 h2; and when one of
the host is not reachable from the other , console was hung or continue to print " waiting for server to be ready message".  Fix involves 2 changes. Replace while loop with if block of code. Replace telnet mechanism with
netcat mechanism. (which reduces the unnecessary 1 minute wait of telnet to few secs).

Fix has been tested for the following :
1. mininet minimal, tree(depth=3,fanout=4 with 64 hosts), linear(16 hosts)
2. iperf between different reachable and unreachable hosts
3. pingall and pingpair