row_cache: Switch _stats.hits/misses to row granularity
row_cache: Switch _stats.hits/misses to row granularity

Those are exported by the RESTful APIs called
"get_row_hits/get_row_misses" and reported by nodetool.

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla