Send warning to user when log file is too big

[vschiavoni]

The command splay-get-log silently tries to fetch a job log, ignoring potential big files.
Possible solutions:
1 - the rcp-server first checks the size of the log against a configuration parameter, and if the log is too big, it sends back to the client a warning about the size
2 - the client could specify an option to force the operation despite its size (--force, -f)
3 - the client asks to first compress the log and then fetch the log_job_X.zip file