DCBs now added/removed without locks

This is not globally safe yet, but all other access is directly or
indirectly related to maxadmin, which is irrelevant as far as
performance testing is concerned.

server/core/poll.cc

@@ -317,7 +317,7 @@ static int add_fd_to_workers(int fd, uint32_t events, MXS_POLL_DATA* data)
 
     ev.events = events;
     ev.data.ptr = data;
-    data->thread.id = 0; // In this case, the data will appear to be on the main thread.
+    data->thread.id = current_thread_id; // The DCB will appear on the list of the calling thread.
 
     int stored_errno = 0;
     int rc = 0;