Changed

- If the relay to measure is an exit, put it in the exit position and choose a
  non-exit to help. Previously the relay to measure would always be the first
hop. (GH#181)
- Try harder to find a relay to help measure the target relay with two changes.
  Essentially: (1) Instead of only picking from relays that are 1.25 - 2.00
times faster than it by consensus weight, try (in order) to find a relay that
is at least 2.00, 1.75, 1.50, 1.25, or 1.00 times as fast. If that fails,
instead of giving up, (2) pick the fastest relay in the network instead of
giving up. This compliments the previous change about measuring target exits in
the exit position.

Fixed

- Exception that causes sbws to fall back to one measurement thread. We first
  tried fixing something in this area with `88fae60bc` but neglected to
remember that `.join()` wants only string arguments and can't handle a `None`.
So fix that.
- Exception when failing to get a relay's `ed25519_master_key` from Tor and
  trying to do `.rstrip()` on a None.
- `earliest_bandwidth` being the newest bw not the oldest (thanks juga0)
- `node_id` was missing the character "$" at the beginning
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