Add HTTP Transporter benchmark.

@xiangli-cmu Here's my HTTP transporter benchmark implementation. This is 100 clients concurretly executing 100 messages:

```
BenchmarkHttpTransporter	       5	 449353064 ns/op
```

The results are tricky to interpret. You have to divide `449353064` by `100*100` which is `44,935 ns/op`. That means a throughput of 22,254 nops per second.