Fix a race condition on nameserver reconfiguration.

Fix a race condition on nameserver reconfiguration.

This resolves bug 526, wherein we would crash if the following
events occurred in this order:
  A: We're an OR, and one of our nameservers goes down.
  B: We launch a probe to it to see if it's up again. (We do this hourly
     in steady-state.)
  C: Before the probe finishes, we reconfigure our nameservers,
     usually because we got a SIGHUP and the resolve.conf file changed.
  D: The probe reply comes back, or times out. (There is a five-second
     window for this, after B has happens).

IOW, if one of our nameservers is down and our nameserver
configuration has changed, there were 5 seconds per hour where HUPing
the server was unsafe.

Bugfix on 0.1.2.1-alpha.  Too obscure to backport.

svn:r18306