Optimise MAVFTP upload to improve performance

[rmackay9]

A Copter-4.4 user reported in this discussion 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 ArduPilot/MAVProxy#1270

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

[rmackay9]

I've changed the title to reflect that this is no longer considered a bug but rather an enhancement request.

On the Nov 14th dev call we discussed with Tridge and discovered that MAVFTP's implementation (both within the GCSs and the flight code) has been optimised for downloading (e.g. from the autopilot->GCS) but not for uploading (e.g. GCS->autopilot).

There were two suggestions for improving peformance:

1. send multiple MAVFTP messages from the GCS to the autopilot without waiting for a response.
2. create a new MAVLink message to send longer messages just like we do for downloading. Currently it seems the MAVFTP messages are only 80bytes long