CDSE: set smaller tile size for processing

[jdries]

We notice that there's some benefits to processing with a smaller tile size.
We previously tried to increase it, but this was because data loading was faster for large tiles.
For the rest of the processing, we see that smaller tiles result in fewer memory issues, so perhaps we can now use a 64px tilesize as default if possible, or else derive it from apply_neighborhood parameters.

[jdries]

We can do something rather general in layercatalog.py:

elif(get_backend_config().default_reading_strategy == "load_per_product"):
            datacubeParams.setLoadPerProduct(True)
            if "tilesize" not in feature_flags:
                #when doing load_per_product, tilesize does not affect read_performance, and smaller chunks are better for memory usage
                getattr(datacubeParams, "tileSize_$eq")(128)

not committing this now, as it requires some followup. Maybe even better is to make this 'default' chunk size a parameter in the backend config or have it as a job option rather than custom feature flag.

[jdries]

configured new default of 128 on dev/staging