cql: prevent overflow when computing averages
cql: prevent overflow when computing averages

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: <c1333407b8d054452a12baa354fed5de5e8f6b8f@scylladb.com>

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