Use UserAuth, instead of WAIT_AUTH in SshClient.

Two things probably worth noteing:
- AuthFuture.getException() already returns non-NULL IFF the channel closed
- the calls such as UserAuthServiceClient.authPassword needed to be changed
  so that they returned a UserAuthFuture containing an exception, instead of
  throwing an exception, when the channel is closed.
this lets calling code iterate using returned AuthFuture values instead of
using WAIT_AUTH.
[SSHD-218] Delete DefaultSshFuture.cancel() et.al.

It turns out that the only call to .cancel() is the circular:
                if (future.isCanceled()) {
                    connectFuture.cancel();