[tablet] --tablet_history_max_age_sec is a runtime flag

The way how --tablet_history_max_age_sec is used in
Tablet::GetTabletAncientHistoryMark() mandates a 'runtime' tag for the
flag.  That allows for using the `kudu tserver set_flag` CLI tool
without adding the --force option when updating the setting.

Change-Id: Ie2b1553325cd33953d104b5bb3e8aeb73d90642a
Reviewed-on: http://gerrit.cloudera.org:8080/19286
Tested-by: Alexey Serbin <alexey@apache.org>
Reviewed-by: Yifan Zhang <chinazhangyifan@163.com>
diff --git a/src/kudu/tablet/tablet.cc b/src/kudu/tablet/tablet.cc
index e60443f..18ca960 100644
--- a/src/kudu/tablet/tablet.cc
+++ b/src/kudu/tablet/tablet.cc
@@ -154,6 +154,7 @@
              "initiated at a snapshot that is older than this age will be "
              "rejected. To disable history removal, set to -1.");
 TAG_FLAG(tablet_history_max_age_sec, advanced);
+TAG_FLAG(tablet_history_max_age_sec, runtime);
 TAG_FLAG(tablet_history_max_age_sec, stable);
 
 // Large encoded keys cause problems because we store the min/max encoded key in the