Rework the types used during data loading

This improves on the previous change. Currently, the node and instance
lists shipped around during data loading are (again) association lists.
For instances it's not a big issue, but the node list is rewritten
continuously while we assign instances to nodes, and that is very slow.
The code was originally written for small (10-20 node) clusters, and
today with multinodes… :)

Rewriting to use Node.List/Instance.List makes a bit of a messy patch,
but it allows to remove some custom functions for assoc. list
processing, and also some custom unittests.

At the end, the load time is almost halved, and we spend time now just
in the string parsing code (which is, as we know, slow…).

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