ability to dynamically repartition an index

Was talking to @jliang00 about cbgt and ideas/concerns/feedback on future requirements:

Dynamic repartitioning of PIndexes would be good.

For example, PIndex ac923 is assigned to cover vbuckets 0-499... then the ask is tosomehow be able to split it into multiple PIndexes, so the vbuckets might be reassigned like....

```
- 0-249
- 249-499
```

Not sure if that means two brand new PIndexes, or if that means PIndex ac923 is redefined plus one new PIndex. Probably the former is simpler due to the existing PIndex identifier hashing.

The issue names no files, tests, or entry points. Start by reading the existing PIndex identifier hashing and assignment behavior, then clarify whether repartitioning creates two new PIndexes or redefines one and creates another. Done means the chosen design supports splitting the example vbucket range without ambiguity.

Written by the indexing model from the issue text.