Close the socket with linger=100 when deleting a ZeroMQHandler

Without this change, a ZeroMQHandler not able to reach the
subscriber will hang indefinitely when deleted. This is due
to the 'ZMQ_LINGER' option which by defaut tells ZeroMQ to
 wait for every pending message to be sent before terminating
a context, without any limit in time.

We limit this pending time to 100ms.
Merge pull request #124 from onitu/zmqhandler_exit

Close the socket with linger=100 when deleting a ZeroMQHandler