This change includes a NewResourceMap API, alongside the existing,
but renamed, NewResourceTree API. This also tidies up the resulting
struct for public consumption. This is required for downstream tests.
Parallelize collection of logs at each layer of the resource tree walk. Since almost all cost of log collection is at leaf nodes, this should allow the total time for log collection to be close to the time taken for the single longest leaf node, instead of the sum of all leaf nodes.