MergePendingChecker createMergePendingChecker(double minMergeCompletionRatio) {
        return new AggregatedStatsMergePendingChecker(getAggregatedStats(), minMergeCompletionRatio);
    }
Class that stores content cluster stats (with bucket space stats per node) for
the current cluster state version.

Each distributor reports bucket space stats for the different content nodes.
These reports arrive with getnodestate RPC calls,
and eventually ends up as calls to updateForDistributor().
No assumptions are made on the sequence of getnodestate calls.
For instance, it's perfectly fine for the calls to arrive in the
following order:
  distributor 0
  distributor 1
  distributor 1
  distributor 0
  distributor 2
  ... etc

@author hakonhall
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/ClusterStatsAggregator.java