sftp.get may fail on rekey of server

``` java
SSHClient client = impl.createClient();
SFTPClient sftp = client.newSFTPClient();
sftp.get(fileName, new FileSystemFile(new File(testDest )));
```

If the fileName is a hugeFile (over 2G), and the server submits a 'rekey' operation, this gets fails with the following logging:

```
INFO net.schmizz.sshj.transport.random.BouncyCastleRandom..48 Generating random seed from SecureRandom.
INFO nl.vpro.nep.service.impl.SSHClientFactory.lambda$create$0.49 Validating sftp-itemizer.nepworldwide.nl with fingerprint 94:06:26:d5:e4:f5:18:b5:52:a9:19:b1:97:db:94:9e
INFO net.schmizz.sshj.transport.TransportImpl.sendClientIdent.214 Client identity string: SSH-2.0-SSHJ_0.24.0
INFO net.schmizz.sshj.transport.TransportImpl.init.178 Server identity string: SSH-2.0-Please re-connect using SFTP
INFO net.schmizz.sshj.connection.channel.direct.SessionChannel.startSubsystem.187 Will request `sftp` subsystem
RROR net.schmizz.sshj.transport.TransportImpl.die.593 Dying because - Broken transport; encountered EOF
net.schmizz.sshj.transport.TransportException: Broken transport; encountered EOF
at net.schmizz.sshj.transport.Reader.run(Reader.java:57)
INFO net.schmizz.sshj.transport.TransportImpl.notifyDisconnect.192 Disconnected - UNKNOWN
RROR net.schmizz.concurrent.Promise.tryRetrieve.174 <> woke to: net.schmizz.sshj.sftp.SFTPException: Broken transport; encountered EOF

net.schmizz.sshj.transport.TransportException: Socket closed

at net.schmizz.sshj.transport.TransportImpl.write(TransportImpl.java:454)
at net.schmizz.sshj.connection.channel.ChannelOutputStream$DataBuffer.flush(ChannelOutputStream.java:102)
at net.schmizz.sshj.connection.channel.ChannelOutputStream$DataBuffer.flush(ChannelOutputStream.java:70)
at net.schmizz.sshj.connection.channel.ChannelOutputStream.flush(ChannelOutputStream.java:182)
at net.schmizz.sshj.sftp.SFTPEngine.transmit(SFTPEngine.java:297)
at net.schmizz.sshj.sftp.SFTPEngine.request(SFTPEngine.java:130)
at net.schmizz.sshj.sftp.RemoteResource.close(RemoteResource.java:53)
at net.schmizz.sshj.sftp.SFTPFileTransfer$Downloader.downloadFile(SFTPFileTransfer.java:149)
at net.schmizz.sshj.sftp.SFTPFileTransfer$Downloader.download(SFTPFileTransfer.java:105)
at net.schmizz.sshj.sftp.SFTPFileTransfer$Downloader.access$300(SFTPFileTransfer.java:90)
at net.schmizz.sshj.sftp.SFTPFileTransfer.download(SFTPFileTransfer.java:71)
at net.schmizz.sshj.sftp.SFTPClient.get(SFTPClient.java:239)
at nl.vpro.nep.service.impl.NEPSSHJDownloadServiceImplTest.testSshj(NEPSSHJDownloadServiceImplTest.java:105)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: java.net.SocketException: Socket closed
at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:118)
at java.net.SocketOutputStream.write(SocketOutputStream.java:155)
at net.schmizz.sshj.transport.TransportImpl.write(TransportImpl.java:451)
... 35 more
```

With older scp (command line clients) this goes wrong in a similar manner, but newer ones work ok. After some investigation we concluded that this rekeying could be the cause (I had never heard of it).