Fixed Issue ros/roslisp#12

For use-case 1:
When subscribing to oneself, subscribers do not wait for answer to send-tcp-header because it would otherwise deadlock. This is applicable because the answer has always been ignored anyway. Additionally, we know the md5 sums to match because we are the same node. Finally, in case of self-subscription publisher do not send response to received tcp-header.

For use-case 2:
When subscribing to oneself, subscribers do not send an xml-rpc to avoid a deadlock. Instead, they directly call |requestTopic| on themselves.
Just a minor comment on Lisp style: I would only use IF if there is just one small expression in the if and else block. For multiple lines, I would always use COND. But that's really just a minor thing.