evas/cserve2: Add a "processing" queue to requests.

evas/cserve2: Add a "processing" queue to requests.
This is a list that holds all requests being processed at the moment. It
can be used so that when a new request is added, first we look in this
queue to avoid creating a new request that is the same as the one being
processed right now. If it is already being processed, a new waiter is
added to it, just like when the request was on its original queue.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/evas@71607 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33