Before this change the code which summed up the existing transfers
over all the stats groups forgot to add the old transfer time and old
transfers in.
This meant that the speed and elapsedTime got increasingly inaccurate
over time due to the transfers being culled from the list but their
time not being accounted for.
This change adds the old transfers into the sum which fixes the
problem.
This was only a problem over the rc.
Fixes#4569
Without the fix we can have a race, example:
```
Write at 0x00c000432039 by goroutine 187:
github.com/rclone/rclone/fs/accounting.(*StatsInfo).Error()
fs/accounting/stats.go:495 +0x3f1
github.com/rclone/rclone/fs/accounting.(*StatsInfo).Error-fm()
fs/accounting/stats.go:477 +0x55
github.com/rclone/rclone/fs/walk.listRwalk.func1()
fs/walk/walk.go:162 +0xd2
github.com/rclone/rclone/fs/walk.walk.func2()
fs/walk/walk.go:402 +0x30f
Previous read at 0x00c000432039 by goroutine 184:
github.com/rclone/rclone/fs/accounting.(*statsGroups).sum()
fs/accounting/stats_groups.go:351 +0xcae
github.com/rclone/rclone/fs/accounting.rcTransferredStats()
fs/accounting/stats_groups.go:132 +0x1f4
```
Fixes#3844
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.
