Local rtree / libspatialindex indexes

[mojodna]

This will take the form of a custom Catalog implementation that either uses files bundled with the function or specified as URLs (that are fetched on function initialization / periodically).

[mojodna]

Per discussion with @iandees, we're going to use SQLite (similar to the terrain tiles build) seeded with pyramid subsets in order to re-use the existing PostGIS catalog SQL.

libspatialindex is still potentially useful for other use-cases.

Either implementation will facilitate taking subsets of imagery offline.