There is a separate DB per core, each serving a subset of the key
space. From the outside in appears to behave as one DB.
item_key type was changed to include the hash so that we calculate the
hash only once. The same hash is used for sharding and hashing. No
need for store_hash<> option on unordered_set<> any more.
Some seastar-specific and hashtable-specific stats were moved from the
general "stats" command into "stats hash", which shows per-core
statistics.
b0dd9e736c