Use the return value of the function, don't shortcut through the current...

Use the return value of the function, don't shortcut through the current thread. sometimes under py3 the current thread id isn't set, and the release() methods we're trying to patch around use get_ident() anyway. Fixes #615. Really, not just the test cases.

gevent/monkey.py

@@ -150,7 +150,10 @@ def patch_time():
 def _patch_existing_locks(threading):
     if len(list(threading.enumerate())) != 1:
         return
-    tid = threading.current_thread().ident
+    try:
+        tid = threading.get_ident()
+    except AttributeError:
+        tid = threading._get_ident()
     rlock_type = type(threading.RLock())
     try:
         import importlib._bootstrap