Fix #349: threaded resolver hangs due to import lock

Patch by @ajdavis. https://github.com/gevent/gevent/issues/349#issuecomment-31665013

Close #350.

gevent/resolver_thread.py

 # Copyright (c) 2012 Denis Bilenko. See LICENSE for details.
 import _socket
-from gevent.hub import get_hub
+from gevent.hub import get_hub, text_type
 
 
 __all__ = ['Resolver']
 
 
+# trigger import of encodings.idna to avoid https://github.com/gevent/gevent/issues/349
+text_type('foo').encode('idna')
+
+
 class Resolver(object):
 
     expected_errors = Exception

greentest/getaddrinfo_module.py

+import socket
+import gevent.socket as gevent_socket
+
+gevent_socket.getaddrinfo(u'gevent.org', None, socket.AF_INET)

greentest/test__getaddrinfo_import.py

+# a deadlock is possible if we import a module that runs Gevent's getaddrinfo
+# with a unicode hostname, which starts Python's getaddrinfo on a thread, which
+# attempts to import encodings.idna but blocks on the import lock. verify
+# that Gevent avoids this deadlock.
+
+import getaddrinfo_module
+del getaddrinfo_module  # fix pyflakes