Make RelayList stop overloading the CPU

Created by: pastly

Make relay list refreshing take much less time by not allowing worker threads to dogpile on the CPU. Before they would all start requesting descriptors from Tor at roughly the same time, causing us to overload our CPU core and make the process take unnecessarily long. Now we let one thread do the work so it can peg the CPU on its own and get the refresh done ASAP.

Closes #205 (closed)