C CLI should close session properly

Whenever you quit from the C command line client, you got a warning on the server logs like:

```
2019-12-11 10:28:40,973 [myid:] - WARN [NIOWorkerThread-6:NIOServerCnxn@364] - Unexpected exception
EndOfStreamException: Unable to read additional data from client, it probably closed the socket: address = /0:0:0:0:0:0:0:1:51026, session = 0x1000012817e001c
at org.apache.zookeeper.server.NIOServerCnxn.handleFailedRead(NIOServerCnxn.java:163)
at org.apache.zookeeper.server.NIOServerCnxn.doIO(NIOServerCnxn.java:326)
at org.apache.zookeeper.server.NIOServerCnxnFactory$IOWorkRequest.doWork(NIOServerCnxnFactory.java:522)
at org.apache.zookeeper.server.WorkerService$ScheduledWorkRequest.run(WorkerService.java:154)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)
```

This happens no matter how you exit from the C command-line client. E.g.:

• using Ctrl-C
• using the `quit` command
• using the `--cmd` option and executing a single command