MOI may use >50% CPU for disk snapshotting

There is an indexer config - indexer.settings.moi.persistence_threads - that is expected to limit the CPU consumed by disk snapshotting. By default it is set to half of GOMAXPROCS(0). This config limits the CPU consumption in 2 ways by controlling:

1) The number of threads allotted to each index for disk snapshotting: This is proportional to the fraction of total node items in that index. Each index should get at least 1 thread. For example, if an index has half the items of the node, it will get half the number of threads, and for an extreme example, if there is an index with 1 item in a node with 1M items, this index gets 1 thread. 
2) The number of indexes for which disk snapshotting can happen in parallel - moiWriterSemaphoreCh. 

So, if there is a mix of large and small indexes, due to the minimum 1 thread, there may be more than intended threads running disk snapshotting.