Fix tee filter

With the addition of filter capabilities, the tee filter should work with
all sorts of routers that require at most the RCAP_TYPE_CONTIGUOUS_INPUT
capability.

Due to a recent discovery of the server's capability to process multiple
requests, the filter can safely send data from one service to another
without waiting for the earlier replies.

This also fixes a minor problem with the cloning of DCBs where the backend
DCBs could end up in the wrong thread's pool.

server/core/dcb.c

@@ -295,6 +295,7 @@ dcb_clone(DCB *orig)
         clonedcb->ssl_state = orig->ssl_state;
         clonedcb->remote = remote;
         clonedcb->user = user;
+        clonedcb->thread.id = orig->thread.id;
         clonedcb->protocol = orig->protocol;
 
         clonedcb->func.write = dcb_null_write;