threadutil: don't limit the number of threads

With a maxsize to the Queue of threads we are introducing a blocking call while
adding new threads.

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/threadutil.py

@@ -85,7 +85,7 @@ class threadlist:
 # Exit-notify threads
 ######################################################################
 
-exitthreads = Queue(100)
+exitthreads = Queue()
 
 def exitnotifymonitorloop(callback):
     """An infinite "monitoring" loop watching for finished ExitNotifyThread's.