sort-of-workaround fix for issue #128: allow re-connection to re-spawned subscriber

This is a minimalistic patch to rospy that fixes issue #128.

This is done here with a simple trick: it extends _TopicImpl.has_connection to only return true when it has a connection object with the specified uri *and also* the corresponding TCP client has not finished. If the corresponding TCP client has already finished then it returns False and makes the client node create a new connection to the publisher.

With this small patch, the test example shown in issue #128 now works for me.