Before this change bisync adjusted the global MaxCompletedTransfers
variable which caused races.
This adds a SetMaxCompletedTransfers method and uses it in bisync.
The MaxCompletedTransfers global becomes the default. This can be
changed externally if rclone is in use as a library, and the commit
history indicates that MaxCompletedTransfers was added for exactly
this purpose so we try not to break it here.