Should more Facets implementations be concurrent?

### Description

Faceting can happen in parallel for each segment, but most faceting implementations don't take advantage of this.

I'm wondering if more faceting implementations should be concurrent. Maybe we can decide to run in sequential mode or concurrent mode dynamically based on the number of docs we facet over.

For an example of concurrent facets, see`ConcurrentSortedSetDocValuesFacetCounts`.

Start with ConcurrentSortedSetDocValuesFacetCounts, the example of concurrent faceting named in the issue, then compare it with the other faceting implementations. Determine which implementations could safely run concurrently and whether execution should be selected dynamically based on the number of documents; done requires a concrete implementation decision.

Written by the indexing model from the issue text.