need to fix perf issue with integer import in importbatch.go

[jaffee]

Detailed in a TODO comment as usual.

		// TODO(jaffee) I think this may be very inefficient. It looks
		// like we're copying the `ids` and `values` slices over
		// themselves (an O(n) operation) for each nullIndex so this
		// is effectively O(n^2). What we could do is iterate through
		// ids and values each once, while simultaneously iterating
		// through nullindices and keeping track of how many
		// nullIndices we've passed, and so how far back we need to
		// copy each item.
		//
		// It was a couple weeks ago that I wrote this code, and I
		// vaguely remember thinking about this, so I may just be
		// missing something now. We should benchmark on what should
		// be a bad case (an int field which is mostly null), and see
		// if the improved implementation helps a lot.

Now I've actually run into it:

		// Update: I ran into this on a largish batch size (4M) with a
		// very small percentage of nils (0.5%) - was very obvious in
		// the CPU profile