collecting usage stats

[fwang2]

It would be nice (for reasons other than whatever), the PCP usage stats are collected so we know how many users, how many bytes have been transferred.

One possible way to address it is:

• After the treewalk, we can write a record (user, commands, total size etc), with a job status as "Failed".
• Upon successful transfer, this record shall be updated with "success".
• The location of the bookkeeping is a bit tricky: for simple case, we can grab from an environment variable.