Start to opentracker's memory requirements

* peer lists for v4 and v6 are now lazily allocated and, more importantly, will be freed once there are no more peers
* The downloaded count is now per torrent and not per peer list anymore, moving in what was alignment loss, anyway
* The (now) per torrent downloaded and the per peer list peer and seed count shrunk from size_t to uint32_t
* A helper guards accesses to the peer and seed count to prevent null pointer dereferences
* The base time also shrank from time_t to uint32_t, minutes since the epoch currently fits in 25 bits

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