Use ExceptionDispatchInfo to retain call stack in AsyncResult.EndInvoke()

[scott-xu]

Trying to understand some flaky tests.
e.g.

  Failed Test_Sftp_Multiple_Async_Upload_And_Download_10Files_5MB_Each [2 s]
  Error Message:
   Test method Renci.SshNet.IntegrationTests.OldIntegrationTests.SftpClientTest.Test_Sftp_Multiple_Async_Upload_And_Download_10Files_5MB_Each threw exception: 
Renci.SshNet.Common.SshConnectionException: Client not connected.
  Stack Trace:
      at Renci.SshNet.Common.AsyncResult.EndInvoke() in /_/src/Renci.SshNet/Common/AsyncResult.cs:line 88
   at Renci.SshNet.SftpClient.EndUploadFile(IAsyncResult asyncResult) in /_/src/Renci.SshNet/SftpClient.cs:line 1165
   at Renci.SshNet.IntegrationTests.OldIntegrationTests.SftpClientTest.Test_Sftp_Multiple_Async_Upload_And_Download_10Files_5MB_Each() in /home/appveyor/projects/ssh-net/test/Renci.SshNet.IntegrationTests/OldIntegrationTests/SftpClientTest.Upload.cs:line 150
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
  Standard Output Messages:
 SSH Server host name: 127.0.0.1

[Rob-Hague]

This one often fails locally for me - in the past I looked at it and concluded the OpenSSH instance in the container was either erroring out or crashing (IIRC), but I did not work out why or whether it's our fault. Presumably it is.