Report PG history synchronously during write

This has 2 effects:
1) OSD sets aren't added into PG history until actual write attempts anymore
   which removes unneeded extra osd_sets in PG history
2) New OSD sets are reported synchronously and can't be lost on PG restarts
   happening at the same time with reconfiguration

src/osd_cluster.cpp

@@ -885,7 +885,6 @@ void osd_t::report_pg_states()
             if (pg.history_changed)
             {
                 // Prevent race conditions (for the case when the monitor is updating this key at the same time)
-                // FIXME: target_history updates may be lost on PG re-peering
                 pg.history_changed = false;
                 std::string history_key = base64_encode(st_cli.etcd_prefix+"/pg/history/"+std::to_string(pg.pool_id)+"/"+std::to_string(pg.pg_num));
                 json11::Json::object history_value = {