Add a `connect` command for testing.
With `./src/zcashd -debug=net -logips=1`:
```
2019-09-19 15:24:38 Added connection to 127.0.0.1:35932 peer=1
2019-09-19 15:24:38 connection from 127.0.0.1:35932 accepted
2019-09-19 15:24:38 socket closed
2019-09-19 15:24:38 disconnecting peer=1
```
With `RUST_LOG="trace"`, `cargo run connect`:
```
Sep 19 08:24:24.530 INFO zebrad::commands::connect: version=Version { version: Version(170007), services: Services(1), timestamp: 2019-09-19T15:24:24.530059300Z, address_recv: (Services(1), V4(127.0.0.1:8233)), address_from: (Services(1), V4(127.0.0.1:9000)), nonce: Nonce(1), user_agent: "Zebra Connect", start_height: BlockHeight(0), relay: false }
Sep 19 08:24:24.530 TRACE Task::run: tokio_executor::threadpool::task: state=Running
Sep 19 08:24:24.530 DEBUG Task::run: tokio_net::driver::reactor: adding I/O source token=0
Sep 19 08:24:24.530 INFO zebrad::commands::connect: version_bytes="24e9276476657273696f6e000000000063000000cb30ab03179802000100000000000000a89d835d00000000010000000000000000000000000000000000ffff7f0000012029010000000000000000000000000000000000ffff7f000001232801000000000000000d5a6562726120436f6e6e6563740000000000"
Sep 19 08:24:24.530 TRACE Task::run: log: registering with poller
```
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