Distance Matrix Partitioning

[kevinmichaelchen]

Google Maps has certain limits:

1. Maximum of 25 origins or 25 destinations per request.
2. Maximum 100 elements per server-side request.
3. 1000 elements per second (EPS).

(The 3rd limitation constrains how much we parallelize in theory — to 40 requests per second — but in practice won't be an issue since first-pass sorting should apply a limit of less than 1000).

Because of these limitations, we'll need to partition matrices. A 1x100 matrix will need to be split up into 4 1x25 matrices.

The distance package will need to be refactored to be more like the geocode package, where all of the complexity (parallelization, matrix partitioning) lives higher up, and the leaf packages are simple and fewer LOC.