parallelize API interaction once #6 is done

[nkuehn]

until the import

importing a large tree (e.g. 500-1200 categories) takes a fairly long time currently because everything is completely sequential. It's not possible in a different way because the sync does not "understand" the tree ordering to do stuff in the right order.
But once #6 is done it could be possible to parallelize API interaction (e.g. in batches per tree depth).

not critical as massively huge category trees aren't the norm.