Removal of properties using an iterator

[gaffer01]

It should be possible to specify that certain properties are not required in the results of a query, and have an iterator filter them out. If complex properties such as a HyperLogLogPlus are not required then removing them early on in the iterator stack will significantly reduce the amount of serialisation, deserialisation and aggregation that later iterators have to perform. It will also reduce the amount of data that needs to be transferred from tablet servers to the client.

[ac74475]

Being able to prune unwanted properties would be particularly beneficial for the Parquet store.