FTS: num_bytes_used_disk_by_root_reclaimable stat value remains the same after indexed data mutations

Build 7.0.0-5127

• Create cluster: {kv} {index, query}

  {fts}

• Perform initial data load using cbworkloadgen into default bucket

./cbworkloadgen -n 172.23.107.35:8091 -r 100 -i 100000 -b default -s 1024 -j -u Administrator -p password

• Build default fts index on default bucket
• Perform additional data load into default:

./cbworkloadgen -n 172.23.107.35:8091 -r 100 -i 40000 -b default -s 1024 -j -u Administrator -p password --prefix inserts

• Check num_bytes_used_disk_by_root_reclaimable stat value

curl -X GET -u Administrator:password http://172.23.107.77:8094/api/nsstats | jq

After additional inserts it remains 0.