Currently, we use type type of the column as the accumulator when we
average it. This can easily overflow, e.g. (2^31-1)+(3) = overflow.
Fix by using __int128 for the accumulator. It's not standard, but
it's way more efficient and simpler than the alternatives.
Inspired by CASSANDRA-12417, but much simpler due to the availability
of __int128.
Message-Id: <20171112173529.30764-1-avi@scylladb.com>
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