This patch fixes a bug that appears because of an incorrect interaction
between counters and hinted handoff.
When a counter is updated on the leader, it sends mutations to other
replicas that contain all counter shards from the leader. If
consistency level is achieved but some replicas are unavailable, a hint
with mutation containing counter shards is stored.
When a hint's destination node is no longer its replica, it is
attempted to be sent to all its current replicas. Previously, if the
cluster did not have the feature HINTED_HANDOFF_SEPARATE_CONNECTION
enabled, storage_proxy::mutate function would be used for the purpose of
sending the hint. It was incorrect because that function treats
mutations for counter tables as mutations containing only a delta (by
how much to increase/decrease the counter). These two types of mutations
have different serialization format, so in this case a "shards" mutation
is reinterpreted as "delta" mutation, which can cause data corruption to
occur.
This patch fixes the case when HINTED_HANDOFF_SEPARATE_CONNECTION is
disabled, and uses storage_proxy::mutate_internal, which treats "shards"
mutation as regular mutations - which is the correct behavior.
Refs #5833.
Tests: unit(dev)
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