accounting: reset bytes read during copy retry - fixes #4178
accounting: reset bytes read during copy retry - fixes #4178

During a copy/sync command, if an operation fails due to a network
issue and is retried, the underlying io.Reader is re-initialised,
but the stats for bytes already read are not reset, leading to incorrect
stats. THis was fixed by resetting the bytes read when an Account is
re-initialized.

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone