--max-backlog controls the queue length.
Add statistics for the check/upload/rename queues.
This means that checking can complete before the uploads which will
give rclone the ability to show exactly what is outstanding.
A deadlock could occur since we have now put a mutex on GetBytes from
StatsInfo.String (s.mu) - progress (acc.statmu) and read (acc.statmu)
- GetBytes (s.mu).
Fix this by giving stringSet its own locking and excluding the call
which caused the deadlock from the mutex in StatsInfo.String.
