Scoreboard updates should happen at regular time intervals

[cus]

Right now, proftd is updating the scoreboard based on the number of transferred blocks or the percentage of the file transfer progress.

Percent based scoreboard updates can be too infrequent for big files and slow link speeds, because it can take a lot of time to transfer a percent of a file. Blocksize based scoreboard updates can be too frequent for fast links, because 10 blocks of data (which is the default value of PR_TUNABLE_XFER_SCOREBOARD_UPDATES) can be transferred in a fraction of a second on a gigatbit link...

Overall, any scoreboard update method dependant on link speed or file size is sub-optimal, because the user expects the scoreboard to update regularly, not too often (in order to reduce the extra load caused by the scoreboard updates), and not too rarely (to get accurate up-to-date information).

Therefore I suggest to make scoreboard updates based on the elapsed time since the last scoreboard update, e.g. update the scoreboard every second. Elapsed time is already queried in pr_throttle_pause() where the updates take place, so it should be fairly easy to change the implementation.