snapshot-service: Add a memory cache of snapshots paths saved

And add a method to query that cache.

https://bugzilla.gnome.org/show_bug.cgi?id=725393
snapshot-service: Update snapshots after one week

https://bugzilla.gnome.org/show_bug.cgi?id=725393
snapshot-service: Add methods to get the snapshot path

This is useful for the new overview that doesn't need the actual
snapshot, but the path of the snapshot when it has been saved in the
cache.

https://bugzilla.gnome.org/show_bug.cgi?id=725393
history-service: Also return the thumbnail time by ephy_history_service_query_urls

https://bugzilla.gnome.org/show_bug.cgi?id=725393
overview: Remove the overview model from the UI process

Now we query the history and snapshot services directly and the model is
used only in the web process. This fixes a race condition when the
overview page is created before the history query has completed leaving
an empty overview.

https://bugzilla.gnome.org/show_bug.cgi?id=725393