[Client/Server] Thumbnail functionality (sampling zoom / dezoom)

[doriancransac]

Implement a feature allowing the user to only retrieve every X stacktrace (orally discussed already, not sure what our conclusion was).

We either use a modulo at insert time in the collector (n-factor, provided via variable in collectorConfig file), or simply skip scans on the primary (ObjectID ) index of mongoDB / retrieve only every n-th result (not sure how that plays out, performance-wise, i.e if that causes a lot of aweful random I/O's..?).