vector engine filter

[mccanne]

This issue builds on #4818 by adding filtering to the vector engine's projection capability.

Here simple filtering of fields (with boolean logic) can be pushed into the vector scanner where a selection vector will be used to select the values of an object that are then projected downstream.

This path should (probably) not be invoked when projection involves the entire value (and the values aren't trivial in size). In this case, the selection vector should be passed to a ZNG scanner that pick only the values needed (this will be done in a separate issue).