Non-trivial merge 'master-5' (see 'modules/stats')

Non-trivial merge 'master-5' (see 'modules/stats')

modules/stats: adapt aggregate stats to the new hierarchical format

Knot Resolver 6 changed the structure of the stats return value, making
it more hierarchical, i.e. the returned object contains nested
"sub-objects", rather than being flat. This commit adapts the new
aggregate stats to this new structure, for consistency.