Refactor the ContentMetadataInspector

[alexander-schranz]

This should only be done once. Best would be in a compilerpass or of doctrine metadata is not available there (not sure when doctrine load its metadata), we should do it once and as seen in #168 there should be some kind of provider implemented or we need to adjust that the single table inheritance work correctly, which was the use case to make this part override able.

We should avoid that any dev will get into contact with the ContentMetadataInspector it should just work correctly in all cases of doctrine configuration.