Right now call to .row() method may create hash on row's cells.
It's counterintuitive to see a const method that transparently
changes something it points to. Since the only caller of a row()
who knows whether the hash creation is required is the cache
reader, it's better to move the call to prepare_hash() into it.
Other than making the .row() less surprising this also helps to
get rid of the whole method by the next patches.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
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