To communicate with the Go program, Unit setup environment variable
named NXT_GO_PORTS with value contains Unit version, stream id to confirm
application is started, and Unit ports information. Go Unit package parses
this string and compares runtime version with compile time version. In case
of parse error or version mismatch, ListenAndServe() returns with the error.
Passing unsafe.Pointers (void *) from Go to C is complicated by an attempt
to make such pointers less unsafe.
A straightforward optimization is to replace 'unsafe.Pointer' with 'uintptr'
(thanks to Xin Huang for the idea: https://stackoverflow.com/a/44826533 ).
As a result, request registry with mutex is gone.
Go request registration should be removed before C request memory freed.
C request address used as a key in Go map. Freed memory can be instantly
reused for other request and older request registration should removed at this
point to avoid collisions.
This allows to use shared memory to communicate with main process.
This patch changes shared memory segment format and breaks compatibility
with older modules.
Do not reuse shared memory segment with different port until this segment
successfully received and indexed on other side. However, segment can be used
to transfer data via the port it was sent at any time.
