Restrict pgrouting query to bounding box

[amillb]

This should improve the performance of 'updateCostMatrix()` and deal with memory issues.

See http://docs.pgrouting.org/latest/en/pgRouting-concepts.html#performance

[amillb]

Let's not do this, because that will slow down queries given the need to select a subset of the streets table each time.

Rather, raise a warning for large streets tables and suggest that the user use a subset of streets.