Add support for remote fetch when there is no common primary key

[ohadzeliger]

When common primary key for an index cannot be derived (e.g. when the PKs for the types are not equal) but the PK length is the same, remote fetch can still be useful as the only thing needed to construct the mapped range is the length of the PK. Add support for this case and remove the commonPrimary Key from the store interface (since it can be calculated based off of the types the index is defined for.