copyViaScp cause I/O error with parallel copying

[qiyuan-wang]

how to reproduce:

1. set a environment no rsync installed;
2. make a ConnectionPool with 2 remotes at least;
3. try copy a folder to remotes' destination.

result:
there should be a fatal error about {foldername}.tmp.tar.gz, 'can not found' or 'can not remove' etc.

reason:
copyViaScp generates tmp.tar file with same folder name. With parallel copying, first call removed the temp file after uploading it to dest, which caused other fail to get the file.

solution:
generates tmp.tar file name with a unique id for each call of copyViaScp.

[gregberge]

@qiyuan-wang will be fixed in #26