Presently lightweight transactions piggy back the old
row value on prepare round response. If one of the participants
did not provide the old value or the values from peers don't match,
we perform a full read round which will repair the Paxos table and the
base table, if necessary, at all participants.
Capture the fact that read optimization has failed in a metric.
Message-Id: <20200304192955.84208-2-kostja@scylladb.com>
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