Optimize ORDER BY clause (physical_orderby)

[Mytherin]

The current ORDER BY implementation has very poor performance because it uses generic types (Value::) which means type checking is done for every single comparison. The performance can be greatly improved by switching to templated comparisons.

[hannes]

Going to have a look here since it impacts window functions. Also ChunkCollection should probably store the last orderbydescription that was used to sort it so that we can avoid re-sorting. This is highly relevant for multiple window functions with same window definition.

[hannes]

The comparison is using templated ops now. Next the scan of the chunk collection according to a sorted vec.