Allow marking all layers as having changed contents

Allow marking all layers as having changed contents, so that we can support updating all layers immediately with high-resolution tiles when pinch zoom finishes. This PR also includes more robust tracking of content_age, which should make bad behavior from the client easier to detect.
The name `mark_all_layer_contents_as_changed_for_layer` seems better, since it's not eliding an apostrophe that can't be written in an identifier.