open all datasets at zonal entry point

[asinghvi17]

this should massively increase performance for lazy=true rasters. But it needs tests with rasters of every form.

[rafaqz]

Yeah probably a lazy test is needed, there may be one already, it can go with GDAL zonal tests

[rafaqz]

We really need chunked burning for this to have much of an effect.

[asinghvi17]

We really need chunked burning for this to have much of an effect.

Doesn't that already work with the exception of #816? Then the chunking only has to be done on the top level...similar to what RangeExtractor does

Also, the main use is to avoid the performance footgun IMO, not necessarily for that. People will get caught in the trap of not using open very easily...

[rafaqz]

I'm pretty sure #814 is fixed now.

But there will be other memory problems from not chunking, although I guess they are with rasterize proper as those paths aren't really hit in zonal.

Ok I'll just merge this if there is a lazy test