Before this change, using -P occasionally deadlocked on the progress
mutex and the stats mutex since they call each other.
This is fixed by shortening the locking window in the progress routine
so as not to include the stats calculation.
Introduce stats groups that will isolate accounting for logically
different transferring operations. That way multiple accounting
operations can be done in parallel without interfering with each other
stats.
Using groups is optional. There is dedicated global stats that will be
used by default if no group is specified. This is operating mode for CLI
usage which is just fire and forget operation.
For running rclone as rc http server each request will create it's own
group. Also there is an option to specify your own group.
Before this fix rclone didn't wait for the stats to be finished before
exiting, so the final new line was never printed.
After this change rclone will wait for the stats routine to cease
before exiting.
