To pass Go object references to C and back we use hack with casting to
unsafe and then to uintptr. However, we should not store such references
because Go not guaratnee it will be available by the same address.
Introducing map with integer key helps to avoid dereference stored address.
This closes#253 and #309 issues on GitHub.
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.
