Each counter cell has a header with an entry for each local and global
shards. The detection of remote shards is done by checking if there are
any counter shards that do not have an entry in the header. This is done
by computing the number of counter shards in a cell and comparing it to
the number of header entries. However, the computation was wrong and
included the size taken by the header itself. As a result, if the header
was as big or larger than a single counter shard Scylla incorrectly
complained about remote shards.
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