vector engine agg

[mccanne]

This issue builds on the vector's filter + projection capability with aggregations and group-bys.

Here filtering and selection of fields will create vectors that are then fed into a native group-by / agg function implementation. For example, instead of sum(x) adding the value of x to a running total for each x encountered, the vector engine will perform a native sum calculation over the vector. When there are group-by expressions, the partial sums will be implemented in a map keyed by the zed.Value. Perhaps we could start with the case where the group-by expressions are simple field references and add general expression support in a subsequent issue. An open question here is how to do this without having to re-serialize native vector values into generic zed.Values.

In this issue we will support only agg functions that are order insentitive (like sum not collect) and we will add support for order sensitive functions in a subsequent issue.