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.
Application timeout limits maximum time of worker response in processing
particular request. Not including the time required to start worker,
time in request queue etc.
Usage:
1. Register handlers in incoming port with nxt_port_rpc_register_handler().
2. Use return value as a stream identifier for next nxt_port_socket_write().
