Move registration of the close_portdbapi_caches() exit hook from the

Move registration of the close_portdbapi_caches() exit hook from the
portage base module to the portree module, so that the base portage
module can be imported without triggering import of the portage.process
(for atexit_register) and portage.dbapi.porttree modules. Also, don't
bother to commit the mtimedb in the exit hook since it's not really
needed.