revert to using threading.Lock

gevent.lock is not present in the 0.13.6 gevent release, which is
what is in Debian wheezy.

authserv/app_nginx.py

-import gevent.lock
 import socket
+import threading
 import urllib
 from flask import Flask, request, abort, make_response
 from authserv.app_common import do_auth
@@ -9,7 +9,7 @@ app = Flask(__name__)
 _default_port_map = {'imap': 143, 'pop3': 110}
 
 _dns_cache = {}
-_dns_cache_lock = gevent.lock.Semaphore()
+_dns_cache_lock = threading.Lock()
 
 def _shard_to_ip(shard):
     hostname = '%s-vpn' % shard