Allow getting per-partition stats for index replicas

Reported in the forum post.

https://forums.couchbase.com/t/index-statistics-api-and-replicas/28944

Possible problem 1:
Some of the partitions are missing in the output.

Steps to reproduce:

1. Setup a cluster with 2 indexer nodes

2. Create a partitioned index with replica.

3. Call /api/v1/stats/bucket-name/index-name on each of the indexer nodes. This does not return stats for all partitions.

Possible problem 2:
As reported in the forum post, the stats returned are can belong to different replica ids when called at different times. Needs further analysis.