Filtering of feature collections

[vvmruder]

I had a deeper look into the code regarding the mechanisms of the OGR and Postgres provider these days. It came to my attention that the properties we can define in the configuration of a resource (docs) are not taken into account in all circumstances. I quickly offered a PR to fix it for the OGR provider.

Currently it seems, we do not have that much reflection of the configured properties in the processes at all. In addition I think it could be an improvement to not remove properties we already fetched from the data source.

In the case where properties are defined in the configuration, my suggestion would be:

• to send them the data source native query if possible. This reduces load and data post processing.
• sanitize all operations (e.g. filters passed via select_properties & properties to the provider, and for operations like create & update this should be reflected too)

First I wanted to ask you opinion on that topic and then I could offer a PR for Postgres and OGR provider to try to fix this.