Throughput example gives error on termination on windows

When I run the throughput example on Windows 8.1, it succeeds the first time but the next times I run it, I get `ERROR:tarpc::future::server: While processing incoming connections: Either the application has not called WSAStartup, or WSAStartup failed. (os error 10093)`

Full log:
```
D:\3rdparty\tarpc>cargo run --example throughput
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target\debug\examples\throughput.exe`
[...]done
TCP: 6701.878581070937MB/s
[...]done
TARPC: 2745.0392500455837MB/s

D:\3rdparty\tarpc>cargo run --example throughput
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target\debug\examples\throughput.exe`
[...]done
TCP: 7711.962606910509MB/s
[...]done
TARPC: 2919.0571906394052MB/s
ERROR:tarpc::future::server: While processing incoming connections: Either the a
pplication has not called WSAStartup, or WSAStartup failed. (os error 10093)

D:\3rdparty\tarpc>cargo run --example throughput
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target\debug\examples\throughput.exe`
[...]done
TCP: 10363.55056035657MB/s
[...]done
TARPC: 3259.7962020236696MB/s
ERROR:tarpc::future::server: While processing incoming connections: Either the a
pplication has not called WSAStartup, or WSAStartup failed. (os error 10093)

D:\3rdparty\tarpc>cargo run --example throughput
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target\debug\examples\throughput.exe`
[...]done
TCP: 8365.178330979292MB/s
[...]done
TARPC: 3025.2071283395167MB/s
ERROR:tarpc::future::server: While processing incoming connections: Either the a
pplication has not called WSAStartup, or WSAStartup failed. (os error 10093)
```