serviceInstanceInventory heartbeat issue

[dzx0525]

hi All,       I am a developer of Genshuixue. I found some performance issue 
when I use Skywalking in production environment.        My Skywalking version 
is 6.1.0. My issues：       Sometimes the service list or instance list will 
lose some data in Skywalking UI. These data will be automatically restored. I 
found that the heartbeat_time in the index of service_instance_inventory are 
not updated.          BRs，        dingzhixiang(丁志翔）