Print progress JSON to stdOUT periodically to help with remote monitoring of CBM

Motivation
Currently the only way to see what progress CBM is making is to look at the progress bar. Ideally we would want a way to be able to remotely and programatically monitor the progress. The initail option would be to use REST but this would require authentication and security complicating things so instead we should just add a flag that allows a JSON object with the progress to be printed to standard out periodically. Then any program that periodically polls standard out can see the progress of the program.

The JSON progress object could have the following structure

{

  "id": 0 # Used to identify which process is outputing this as multiple CBM could be running at the same time

  "percentage": 100 # Global percentage progress

  "size": 10000 # Bytes transferred

  "buckets": [

     "name": "default"

     "percentage": 0

     # Could also add mutations/deletions failed/succeeded

     # Could add stats about number of bytes transferred.

   ]

}