[Optimization] Profile and optimize https://api.outbreak.info/genomics/prevalence-by-location

[flaneuse]

1. Develop a test based on the test API index (https://test.outbreak.info/) to ensure any changes to the handler does not change the output of the data, either in structure or content.
2. Create baseline statistics for the performance of the API based on a set of parameters (testing different pangolin_lineages and other parameters).
3. Profile the handler to determine the rate-limiting steps
4. Develop approaches to optimize the query time.
5. Quantify the improvement in performance.

Current documentation on the endpoints is available at https://github.com/outbreak-info/outbreak.api/blob/master/web/handlers/genomics/docs.md

See also #62, #63, #65, #66

[flaneuse]

Related endpoint: https://api.outbreak.info/genomics/global-prevalence -- should be the same code, just without location filtering.

[flaneuse]

Related to Al's PR: #53