Reader threads keep prevent programs from exiting

The client requires explicit call of `close()` before a program finalization. Otherwise the application hangs up with these threads:

```
"sftp reader" #13 prio=5 os_prio=0 tid=0x00007feedc730ea0 nid=0x2517 in Object.wait() [0x00007fee9a8ba000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000083c09c70> (a net.schmizz.sshj.common.Buffer$PlainBuffer)
at java.lang.Object.wait(Object.java:502)
at net.schmizz.sshj.connection.channel.ChannelInputStream.read(ChannelInputStream.java:103)
- locked <0x0000000083c09c70> (a net.schmizz.sshj.common.Buffer$PlainBuffer)
at net.schmizz.sshj.sftp.PacketReader.readIntoBuffer(PacketReader.java:51)
at net.schmizz.sshj.sftp.PacketReader.getPacketLength(PacketReader.java:59)
at net.schmizz.sshj.sftp.PacketReader.readPacket(PacketReader.java:75)
at net.schmizz.sshj.sftp.PacketReader.run(PacketReader.java:87)

"reader" #11 prio=5 os_prio=0 tid=0x00007fb88474fc20 nid=0x25ee runnable [0x00007fb8512b5000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
at java.net.SocketInputStream.read(SocketInputStream.java:171)
at java.net.SocketInputStream.read(SocketInputStream.java:141)
at net.schmizz.sshj.transport.Reader.run(Reader.java:49)
```

Taking into account that they are reader threads would be great to daemonize them or something like that to prevent the unintuitive behavior.