Make Infinispan use its own thread pool for sending messages in order to avoid thread deadlocks

• when an OOB thread sends a sync request it blocks waiting on a sync in jgroups RequestCorrelator
• it gets released by an another OOB thread when the remote node responds

Now if all the OOB threads are blocked in sending, then there's no available OOB thread to unblock them even if responses from remote nodes have arrived - deadlock. In order to avoid this deadlock we can use a different thread pool for sending OOB messages.

For a discussion around this please refer to: http://infinispan.markmail.org/search/#query:%20list%3Aorg.jboss.lists.infinispan-dev+page:1+mid:7buuwljp7i7fvnpe+state:results