Unique constraint on an index should throw an error if an item already exists

This came up in https://github.com/hashicorp/consul/pull/1463 where I found I could create multiple entries by name, even though there's a unique constraint on that index.

MemDB should throw an error if an item is being inserted into an index marked unique, and there's an existing item there, and that item's primary key doesn't match the incoming one.

Start with the MemDB insertion and unique-index handling described in the issue, then trace how an existing item's primary key is compared with the incoming item. Done means inserting a duplicate unique-index value with a different primary key returns an error, while the same primary key remains valid.

Written by the indexing model from the issue text.