Compute multiple aggregations in one iteration of the match-set

### Description

When a user knows that they want multiple different aggregations, they have to iterate the match-set once for each aggregation, which [is inefficient](https://lists.apache.org/thread/dfw1knm6xqx03wmzq7c9wcxv4loqdg6p). We can remedy this by enabling users to request more than one aggregation in a faceting call and then by handling all the computations in one go.

Start by tracing the faceting call and the match-set iteration that currently computes one aggregation at a time. Determine how multiple aggregation requests should be represented and processed together. Done means a faceting call can request multiple different aggregations and the match-set is iterated only once for their computation.

Written by the indexing model from the issue text.