When use builtin SSH server with concurrent operations, there are probabilities
One connection could use the command from another connections.
Fix this by set SSH_ORIGINAL_COMMAND for each command, not set in global scope.
According to the docs [1], the Reply method must be called for all
requests where WantReply is true. This fixes a hanging java ssh
implementation (jsch) which sets WantReply flag and waits for reply from
the server.
[1] https://godoc.org/golang.org/x/crypto/ssh#Request.Reply
