Allow SSL connections to send keep-alive messages

This seems to prevent mysterious hangs with SSL imap servers (especially
gmail?) and does not harm in any case. So let us enable keep-alive
messages for ssl connections.
Our thread pool should be made more robust against closed SSL
connections (which do not always seem to raise Exceptions), and not
deadlock while waiting for resources or data that will never arrive.

Reviewed-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Gábor Melis <mega@retes.hu>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

Changelog.draft.rst

@@ -23,6 +23,8 @@ Changes
 Bug Fixes
 ---------
 
+* Allow SSL connections to send keep-alive messages.
+
 
 Pending for the next major release
 ==================================

offlineimap/imaplibutil.py

@@ -110,6 +110,10 @@ class WrappedIMAP4_SSL(IMAP4_SSL):
             # FIXME
             raise socket.error(last_error)
 
+        # Allow sending of keep-alive message seems to prevent some servers
+        # from closing SSL on us leading to deadlocks
+        self.sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
+
         #connected to socket, now wrap it in SSL
         try:
             if self._cacertfile: