PM-2311 added support for clustered collections on _id, we might want to add support for clustering on other keys in the future.

Known issues with clustering on other fields:

• Bounded collection scans assume flat types (it's not possible to store arrays on _id) : https://github.com/10gen/mongo/blob/0527f6a2d024a0b0b6ac6706dc82820c405dad9d/src/mongo/db/query/planner_access.cpp#L297-L305