ref: optimize chunks download and fix flamegraph aggregation

[viglia]

This PR aims at improving/addressing the following:

• chunks download optimization: if in the list of ContinuousProfilesCandidate that we receive we have multiple candidates with the same chunkID we should download the same chunk only once.
• fix aggregation bug: currently, if we receive multiple candidates with the same chunkID and (partially or completely) overlapping intervals, we'll end-up double counting certain functions (both for flamegraph and metrics).