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A suggestion to make role propagation simpler from serg@mariadb.org.

Instead of gathering the leaf roles in an array, which for very wide
graphs could potentially mean a big part of the whole roles schema, keep
the previous logic. When finally merging a role, set its counter
to something positive.

This will effectively mean that a role has been merged, thus a random pass
through roles hash that touches a previously merged role won't cause the problem
described in MDEV-12366 any more, as propagate_role_grants_action will stop
attempting to merge from that role.