Implement PG epochs to prevent the "version split"

The "version split" is when:
- A block is written to 1 OSD out of 3, all of them die
- OSDs 2 and 3 come up, the same block is written to both of them
- The remaining OSD comes up. Now all 3 OSDs have the same version of the same object,
  but with different data.

etcd_state_client.h

@@ -23,6 +23,7 @@ struct pg_config_t
     bool pause;
     osd_num_t cur_primary;
     int cur_state;
+    uint64_t epoch;
 };
 
 struct json_kv_t
@@ -48,7 +49,8 @@ struct etcd_state_client_t
     std::function<void(json11::Json::object &)> on_load_config_hook;
     std::function<json11::Json()> load_pgs_checks_hook;
     std::function<void(bool)> on_load_pgs_hook;
-    std::function<void(uint64_t)> on_change_osd_state_hook;
+    std::function<void(pg_num_t)> on_change_pg_history_hook;
+    std::function<void(osd_num_t)> on_change_osd_state_hook;
 
     json_kv_t parse_etcd_kv(const json11::Json & kv_json);
     void etcd_call(std::string api, json11::Json payload, int timeout, std::function<void(std::string, json11::Json)> callback);