Go package build was broken by change 365:28b2a468be43.

Previously, stored configuration wasn't reread on controller
process restart, which resulted in segmentation fault.

- Main process should be connected to all other processes.
- Controller should be connected to Router.
- Router should be connected to Controller and all Workers.
- Workers should be connected to Router worker thread ports only.

This filtering helps to avoid unnecessary communication and various errors
during massive application workers stop / restart.

                          "All problems in computer science can be
                           solved by another level of indirection"

                                                   Butler Lampson

Completion handlers for application response buffers executed after
sending the data to client.  Application worker can be stopped right
after send response buffers to router.  Worker stop causes removal
of all data structures for the worker.

To prevent shared memory segment unmap, need to count the number of
buffers which uses it.  So instead of direct reference to shared
memory, need to reference to intermediate 'handler' structure with
use counter and pointer to shared memory.

Two different router threads may send different requests to single
application worker.  In this case shared memory fds from worker
to router will be send over 2 different router ports.  These fds
will be received and processed by different threads in any order.

This patch made possible to add incoming shared memory segments in
arbitrary order.  Additionally, array and memory pool are no longer
used to store segments because of pool's single threaded nature.

Custom array-like structure nxt_port_mmaps_t introduced.