FlightPlanner : tweak prefetch, Resolves #2591, resolves #2483

[Godeffroy]

When prefetching, do not download tile if file already exist
Add minZoom input in prefetch (like maxZoom)

Use Parallel.For instead of For to get tiles faster !

Using original for loop to download world from zoom 5 to 1 :

Middle of land at zoom 12 to 10 :

Using Parallel.For loop to download world from zoom 5 to 1 :

Middle of land at zoom 12 to 10 :

[meee1]

there is a potential issue here
and that's with googles TOS
300 QPM (requests per minute) per IP address.

the check for existing file is all ok. its just the parallel for that's the problem

[Godeffroy]

I understand the potential issue, but without modification, we can already get more than 300 tiles per minutes :

I think in the TOS, the limit is 30 000 ?

I could limit the parallelism with the option MaxDegreeOfParallelism = 4 ?

[Godeffroy]

I removed the parallel for (and also rebased on master).

[meee1]

whitespace.