Computations with a site_model.xml may become slow

[Michele Simionato]

We saw in the case of Hannover RE Germany computation that the filtering was becoming very slow; the culprit is
the section:

            s_sites = src.filter_sites_by_distance_to_source(
                hc.maximum_distance, hc.site_collection
            ) if hc.maximum_distance else hc.site_collection

hc.site_collection is a property that in presence of a source model performs heavy queries on the db.
We saw on Wilson a load of 180 (!)

[Michele Simionato (michele-simionato)]

We can mitigate the problem by caching the site collection, so that the queries will be run only once per worker, not once per task.
See https://github.com/gem/oq-engine/pull/1392