CAS-10150 Reduce memory usage by MSMetaData hwen computing
subscan props.
It is becoming more common for MSes to have several of tens
of millions or even hundreds of millions of rows. So,
holding several entire columns in memory simultaneously can
be very taxing. The somewhat arbitrary limit I've chosen of
10M row blocks seems to put a reasonable cap on memory
usage with minimum impact on performance. I may modify it
in the future, possibly even letting the client specify it
(directly or indirectly) as problematic use cases are
identified, but for now, it mitigates the most egregious
issues with very large MSes.
