When LSA reclaimer cannot reclaim more space by compaction, it
will reclaim data by evicting from evictable regions.
Currently the only evictable region is the one owned by the row cache.
_lru_len may get stale when row_cache instance goes out of scope
purging all its partitions from cache. I'm assuming we're not really
interested in the number of partitions here, but rather a measure of
occupancy, so I applied a simple fix of using LSA region occupancy
instead.
row_cache class is meant to cache data for given table by wrapping
some underlying data source. It gives away a mutation_reader which
uses in-memory data if possible, or delegates to the underlying reader
and populates the cache on-the-fly.
Accesses to data in cache is tracked for eviction purposes by a
separate entity, the cache_tracker. There is one such tracker for the
whole shard.
