Use the infrastructure introduced in SERVER-111585  to compute NDVs of joining fields and provide this information to the join cardinality computation module.

As we are doing this work, we should consider how it interacts with SERVER-112198. We should also try to detect cases where we are trying to estimate the NDV of a field with a unique index on it. In that case, we can short-circuit and return the collection size rather than looking at the sample and using NR iteration.