tor-netdir: Only select nodes that haven't been used for lower-numbered replicas.

This implements the part of the spec that says a node that has already been selected for a lowered-numbered replica, shouldn't be considered when choosing spread nodes for any other higher-numbered replicas.

Note: previously, the test added in this MR wouldn't have passed (because NetDir::hs_dirs used to return duplicate relays under some circumstances).

This MR contains a few other minor fixes (each contained in a separate commit).

Part of #960 (closed)