accounting: fix time to completion estimates
accounting: fix time to completion estimates

Previous to this change package used for this
github.com/VividCortex/ewma took a 0 average to mean reset the
statistics.  This happens quite often when transferring files though a
buffer.

Replace that implementation with a simple home grown one (with about
the same constant), without that feature.

olihey/rclone,olihey/rclone,B4dM4n/rclone,ncw/rclone,ncw/rclone,olihey/rclone,ncw/rclone,B4dM4n/rclone,olihey/rclone,B4dM4n/rclone,ncw/rclone,B4dM4n/rclone