And adjust callers. The factory functions just sprinkle upgrade_to_v2()
on returned readers for now.
One test in row_cache_test.cc had to be disabled, because the upgrade to
v2 wrapper we now have over cache readers doesn't allow it to directly
control the reader's buffer size and so the test fails. There is a FIXME
left in the test code and the test will be re-enabled once a native v2
reader implementation allows us to get rid of the upgrade wrapper.
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