Fix cache key used by SiteSQLStore.

Fix cache key used by SiteSQLStore.

This makes sure that SiteSQLStore uses a cache key that includes
information about the serialization structure as well as where the
list of sites was loaded from.

This avoids problems with loading "old" serialized versions from the
cache after an upgrade. It also avoids cache conflicts with site lists
loaded from different places.

Change-Id: I7ad5b8ae63dc24598d41b2f150df7c14105d9f62