This fixes memory and shm file descriptor leakage that occurred when a large
request body was passed via shared memory. The leakage was caught with the
"test_settings_body_buffer_size" test. The main condition is the
"body_buffer_size" value exceeding 10 Mb (a shm segment). Thus, the router was
forced to split the body into several shm segments, but these buffers were not
freed because of dummy completion handlers.
This is the port shared between all application processes which use it to pass
requests for processing. Using it significantly simplifies the request
processing code in the router. The drawback is 2 more file descriptors per each
configured application and more complex libunit message wait/read code.
To avoid closing the body fd prematurely, the fd value is moved from
the request struct to the app link. The body fd should not be closed
immediately after the request is sent to the application due to possible
request rescheduling.
