Rescheduling of pending request after configured timeout.

New optional configuration parameter introduced: limits.reschedule_timeout.
Default value 1 second.  In the case when request is written to the port
socket 'in advance', it is called 'pending'.

On every completed request, the head of pending request is checked against
reschedule timeout.  If this request waiting for execution longer than
timeout, it is cancelled, new port selected for this request.

src/nxt_port_rpc.c

@@ -314,10 +314,6 @@ nxt_port_rpc_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
 
     reg = lhq.value;
 
-    if (reg->peer != -1) {
-        nxt_assert(reg->peer == msg->port_msg.pid);
-    }
-
     if (type == _NXT_PORT_MSG_RPC_ERROR) {
         reg->error_handler(task, msg, reg->data);