Use frag_percent, avg_item_size from indexer stats rather than ns_server calculating the values

Currently, ns_server is using it's own calculation for computing "frag_percent" and "avg_item_size" values. Both these stats are dependent on data_size stat value. Any change to indexer data_size stat will impact the "frag_percent" and "avg_item_size" values. Also, "frag_percent" depends on "disk_size" value which has been changed as a part of MB-31787. This led to reporting incorrect fragmentation percentage on the UI (MB-36613)

Instead of ns_server calculating these values, please use the values from indexer stats. Indexer periodic stats already exposes "frag_percent" stat. From build 6.5.0-4750, another stat "avg_item_size" is also exposed