When converting an index to unique/non-unique, we need to call alter on wiredtiger to change the index table's metadata. This is a no-op in the in-memory engine right now so the collMod command will not work correctly.

We should fail the command early for in-memory storage engine.

As part of the ticket, we should also investigate the implication in replication when there are a mix of nodes running on-disk and in-memory storage engine.