fix: only flush the pipeline after executemany if returning

fix: only flush the pipeline after executemany if returning

This has the side effect of breaking rowcount, as before, but can send
many executemany in the same pipeline.

`rowcount` is correct if `returning=True` is set instead, which is a
thing we can at least document, and makes sense: "if you want a result,
you flush the pipeline, dude".