Currently the CollectionMetadata used for filtering contains copy of a subset of the ChunkManager's map of chunk ranges. This is only used for the range deleter and for figuring out when the last chunk is being moved out of a shard.

In order to make the CollectionMetadata a thin wrapper around the ChunkManager, we should remove these usages of CollectionMetadata::_chunksMap and replace them with calls to the ChunkManager itself.

As part of this ticket we should also consider giving CollectionMetadata a by-value semantics.