When loading index from BigTable MT may load an old version of LastUpdate property

[replay]

We regularly update the .LastUpdate property of metric definitions in the big table index when new data points get ingested. But when restarting MT it may happen that an old version of LastUpdate gets loaded which was supposed to already be over-written at that point.
This is probably due to the fact that BigTable keeps all versions around until the next compaction runs, so we need to find a way to make sure it only loads the last version and not any version since the last compaction.