tor-netdir: Select hsdirs from the ring for the specified TP.

Instead of returning hsdirs from multiple rings, hs_dirs now only returns hsdirs from the ring that corresponds to the specified time period.

Also, hs_dirs had a bug in the node deduplication logic: instead of deduplicating the replicas selected for a given time period, the deduplication applied across time periods (because selected_nodes was not reset when the current replica number reached 1). This commit implicitly fixes this bug (because hs_dirs now no longer returns hsdirs from multiple rings).

Closes #960 (closed)