Optimise MAVFTP upload to improve performance

A Copter-4.4 user reported in [this discussion](https://discuss.ardupilot.org/t/terrain-data-loading-on-sd-card-via-mavftp-is-slow/107750/3) that manually uploading a terrain file (produced using https://terrain.ardupilot.org/) using MP's MAVFTP screen is extremely slow. In my tests it was able to upload only 1MB of a 30MB file in 5min.

Performance was the same whether Copter-4.4 or 4.5.0-dev was used. I tested on a CubeOrangePlus.

Surprisingly though MP does better than MAVProxy which fails completely (at least on windows). Here is the corresponding MAVProxy issue https://github.com/ArduPilot/MAVProxy/issues/1270

In case it helps, these messages scrolled by on the MP console a lot.


Start at Mission Planner's MAVFTP screen and reproduce the report by uploading a 30MB terrain file produced by terrain.ardupilot.org, while observing the MP console messages. Compare the transfer behavior with the reported 1MB-in-5-minutes result and the linked MAVProxy issue. Done means the MAVFTP upload completes substantially faster and reliably.

Written by the indexing model from the issue text.