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.
nxt_port_mmap_t stored in arrays and it is unsafe to store
pointer to array element.
Shared memory structures and macros moved to separate header
file to be used by GO package.
