Improve CommandManager

[mgermain]

Most of this really need to be tought in parallel with the JobManager #91.

• Only give the base path to the command manager and let it handle the actual file naming.
• Add a Command class to clean up the code. The class could be a read only string that return the actual command. It could also contain the UID and the log path for the command (might go in the JobManager).
• Add one logfile per command in /logs/logs/worker/command/{COMMAND_UID}.info which will contain information about the command known by the worker. At what time it was started/resumed, on which node, it ran for how long.
• Implement a {job_id}_running_commands.txt (one per job instead of a global one) to allow an epilogue script that move everything to stopped_commands when the job is killed.

[mgermain]

We could also store all the info relative to commands and/or jobs inside an SQLite database.