Mutations on a collection reflecting on num_docs_pending of index of another collection

As Maintenance stream is on bucket level. Mutations on one collection is reflecting on num docs pending on indexes of another collection on same bucket.

Forum Link: https://forums.couchbase.com/t/index-mutations-per-collection/32909/4.

Repro Scenario:

• Have two collections c1, c2 on bucket b1 with two indexer nodes n1, n2
• Create index idx1 on c1
• Create index idx2 on c2 and wait for all indexes to be ready
• Adding mutations on c2 will show mutations pending on idx1 also.