Create thread specific zombie queues

Because each thread has their own epoll file descriptor and only one
thread can process a DCB, it makes sense to move to a per thread zombie
queue. This removes one of the last restrictions on scalability.

server/core/poll.c

@@ -450,6 +450,7 @@ poll_remove_dcb(DCB *dcb)
      */
     dcbfd = dcb->fd;
     spinlock_release(&dcb->dcb_initlock);
+
     if (dcbfd > 0)
     {
         rc = epoll_ctl(epoll_fd[dcb->owner], EPOLL_CTL_DEL, dcbfd, &ev);
@@ -842,12 +843,11 @@ poll_set_maxwait(unsigned int maxwait)
 static int
 process_pollq(int thread_id, struct epoll_event *event)
 {
-    int found = 0;
     uint32_t ev = event->events;
     unsigned long qtime;
 
     DCB *dcb = event->data.ptr;
-
+    ss_dassert(dcb->owner == thread_id);
 #if PROFILE_POLL
     memlog_log(plog, hkheartbeat - dcb->evq.inserted);
 #endif