db::commitlog::replay_position: added std::hash<replay_position>

It's needed for hinted handoff.

Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>

db/commitlog/replay_position.hh

@@ -42,6 +42,7 @@
 
 #include <stdint.h>
 #include "utils/UUID.hh"
+#include "utils/hash.hh"
 
 
 namespace db {
@@ -134,3 +135,12 @@ private:
 std::ostream& operator<<(std::ostream& out, const replay_position& s);
 
 }
+
+namespace std {
+template <>
+struct hash<db::replay_position> {
+    size_t operator()(const db::replay_position& v) const {
+        return utils::tuple_hash()(v.id, v.pos);
+    }
+};
+}