locator: Make sharder accessible through effective_replication_map

For tablets, sharding depends on replication map, so the scope of the
sharder should be effective_replicaion_map rather than the schema
object.

Existing users will be transitioned incrementally in later patches.

locator/abstract_replication_strategy.cc

@@ -134,6 +134,10 @@ std::unique_ptr<token_range_splitter> vnode_effective_replication_map::make_spli
     return locator::make_splitter(_tmptr);
 }
 
+const dht::sharder& vnode_effective_replication_map::get_sharder(const schema& s) const {
+    return s.get_sharder();
+}
+
 const per_table_replication_strategy* abstract_replication_strategy::maybe_as_per_table() const {
     if (!_per_table) {
         return nullptr;

locator/abstract_replication_strategy.hh

@@ -213,6 +213,14 @@ public:
     /// Returns a token_range_splitter which is line with the replica assignment of this replication map.
     /// The splitter can live longer than this instance.
     virtual std::unique_ptr<token_range_splitter> make_splitter() const = 0;
+
+    /// Returns a sharder which reflects shard replica assignment of this replication map.
+    /// The sharder is valid as long as this instance is kept alive.
+    virtual const dht::sharder& get_sharder(const schema& s) const = 0;
+
+    shard_id shard_of(const schema& s, dht::token t) const {
+        return get_sharder(s).shard_of(t);
+    }
 };
 
 using effective_replication_map_ptr = seastar::shared_ptr<const effective_replication_map>;
@@ -274,6 +282,7 @@ public: // effective_replication_map
     std::optional<inet_address_vector_replica_set> get_endpoints_for_reading(const token& search_token) const override;
     bool has_pending_ranges(inet_address endpoint) const override;
     std::unique_ptr<token_range_splitter> make_splitter() const override;
+    const dht::sharder& get_sharder(const schema& s) const override;
 public:
     explicit vnode_effective_replication_map(replication_strategy_ptr rs, token_metadata_ptr tmptr, replication_map replication_map,
             ring_mapping pending_endpoints, ring_mapping read_endpoints, size_t replication_factor) noexcept

locator/tablets.cc

@@ -202,6 +202,7 @@ size_t tablet_metadata::external_memory_usage() const {
 
 class tablet_effective_replication_map : public effective_replication_map {
     table_id _table;
+    tablet_sharder _sharder;
 private:
     gms::inet_address get_endpoint_for_host_id(host_id host) const {
         auto endpoint_opt = _tmptr->get_endpoint_for_host_id(host);
@@ -228,6 +229,7 @@ public:
                                      size_t replication_factor)
             : effective_replication_map(std::move(rs), std::move(tmptr), replication_factor)
             , _table(table)
+            , _sharder(*_tmptr, table)
     { }
 
     virtual ~tablet_effective_replication_map() = default;
@@ -301,6 +303,10 @@ public:
         };
         return std::make_unique<splitter>(_tmptr, get_tablet_map());
     }
+
+    const dht::sharder& get_sharder(const schema& s) const override {
+        return _sharder;
+    }
 };
 
 size_t tablet_aware_replication_strategy::parse_initial_tablets(const sstring& val) const {