Test if adding lots of gc() statements helps memory

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See c9c2166

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Yes, it does constrain memory usage.

However, it adds almost a whole second to each batch.

20 batches of 100 rows took 30s with two gc()'s, 15s with one gc(), and 0.19s with no gc().

If all else fails, it might be ok, with much larger batches. It will almost certainly be the limiting factor.