Improve SingleSiteRequest.sitesUnreachable()

When JGroups tries to send a message to an offline site, it triggers a Event.SITE_UNREACHABLE event. 

This event does not contain any information of what message originated it (could it have? I'm not sure) so, in Infinispan, we iterate over all pending messages and abort the ones going to the affected site.

sitesUnreachable() creates a response to give it to the ReponseCollector. Currently, it is invoked for each Event.SITE_UNREACHABLE event received but we can improve the situation by invoking the method only once.