Fix a longstanding dnsworker-related bug: when we decide to kill a DNS worker for having been in use too long, and we independently decide to close one of the exit connections waiting for that DNS worker (perhaps because we got an END relay cell), it was possible for us to try to double-free the connection.  We were smart enough to detect this and assert, but not smart enough to fix this bug before now.  This should resolve bug 390.  Backport candidate.


svn:r9581