A restart-safe counter. For an unpartitioned index, this a single counter
for the entire index with an initial value of zero (0) and it is stored
in the index Checkpoint record. For a partitioned index, there is a
distinct counter for each index partition, the partition identifier is
used as the high int32 bits of the counter, and the low int32 of the
counter has an initial value of zero (0) in each index partition.