ssize_t is not standard C++. It is a POSIX extension. Therefore, it does
not belong in generic code.

This change tweaks the logic in DBIter to remove the need for signed
integers, so ssize_t can be replaced with size_t. The impacted method
and private member are renamed to better express their purpose.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211471606