Fix target_v3 to not factor in "inactive" hotspots

[vihu]

blockchain_poc_target_v3 module uses blockchain_ledger_v1:get_hexes_list which returns the raw count of hexes (in the region regardless of their POC activity); this results in a targeting a lot of potentially stale hotspots.

Proposed Solution:

• Make h3dex data structure garbage collect "inactive" hotspots. This would naturally filter out inactive hexes and we won't have to modify other pieces.