[Feature Request] Create separate log for each sync/copy job

Hi @ncw,
problem: I am using rclone rcd and calling api sync/copy for copy operation. Suppose I have thousands of sync/copy jobs and some job got failed then it very tedious and nearly impossible to find out the problem in log file where all everything is logged by rclone.

We want to log file for each job separately, sync/copy api should have parameter where we can pass logfilepath(var/log/x.log) and it should create a separate log file(var/log/x_jobid.log) where rclone will log data related to this job.

This is a very good feature to have in rclone, it will be very handy and useful when something is failing for particular job. if we have separate log file for each copy job then it will be easy to identify the problem and to troubleshoot.