Loader functions: move from assoc lists to maps

When loading big clusters, the association lists become a bit slow, so
we'll replace this with a simple Map String Int; the change is trivial
and can be reverted easily, while it brings up a good speedup in the
data loading.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>