Add support for subpools grouping or adding as children to other subpools

[dkormalev]

In some cases it is beneficial to group few subpools together so they would each be restricted by their own limits and also will be restricted together.
For example, we have 500 subpools of 2 concurrent jobs each. We don't want to run more than 10 jobs across all subpools. Grouping all 500 of them and setting the limit to 10 would help in this case. Alternative way is to make subpools a tree (need to be checked if will not add too much extra work for cases when it is not needed).