core: Unload the modules and cached samples before unref'ing the core.

core: Unload the modules and cached samples before unref'ing the core.

As various modules can subscribe to unlink callbacks unloading some modules
may trigger hooks in other modules.

The callbacks associated with these hooks could in turn need to use the core
in some capacity (e.g. perhaps they are module loading modules
(e.g. *-discover, filter-apply or gconf etc. and need to use the core to
unload modules they've loaded).

This change simply ensures that all modules and cached samples are unloaded
before freeing the core.