Count mismatch between curr_items and collections' num_items

Build: 7.1.0-2284

Scenario:

• 5 node cluster
• 3 couchstore buckets (5 scopes per bucket and 40 collections / scope) 

  -------------------------------------------------------------------------------------+

  | Bucket  | Storage Backend | Replicas | Durability | TTL | Items  | RAM Quota   | RAM Used   | Disk Used  | ARR |

  -------------------------------------------------------------------------------------+

  | bucket1 | couchstore      | 0        | none       | 0   | 3000   | 1000.00 MiB | 55.32 MiB  | 92.04 MiB  | 100 |

  | bucket2 | couchstore      | 0        | none       | 0   | 3000   | 1000.00 MiB | 55.15 MiB  | 101.29 MiB | 100 |

  | default | couchstore      | 0        | none       | 0   | 380000 | 7.32 GiB    | 171.16 MiB | 143.26 MiB | 100 |

  ----------------------------------------------------------------------------------------------

• Update bucket replica=3 for all buckets and perform rebalance
• Drop few collections and perform doc_loading in parallel to rebalance

Observation:

Seeing stats mismatch in total_items count in UI and cbstats collections' num_items stats

For example for bucket "bucket2",

UI reports - 32305

Collections items cbstats - 32030

Aggregate of all collections' items value.

.155-10588

.211-10757

.212-10685