Introduce a threshold to the asc/desc sorting criteria

[Kerollmops]

The current Asc/Desc criteria choose between two ways of doing the job:

1. If the number of candidates is lower or equal to 1000, it does a simple sort.
2. If the number of candidates is bigger than 1000, it uses the facet database.

This number (1000) should better be a percentage of the total number of documents. It could be something like 0.1%. This way, we ensure that the best technique is used even if the database grows. #48 helps in retrieving the number of documents and, therefore, speedups computing a percentage.