Add header_register_callback builtin support

[PetrShumilov]

Add compatible with PHP 7.4.0 support of header_register_callback builtin.

Related issues: KPHP-1860

NOTE:

• After discussion with @DrDet we've decided to prohibit exceptions throwing from callback
• f$send_http_103_early_hints shouldn't be a cause of callback invocation

[DrDet]

The main idea behind header_register_callback is to give opportunity to modify headers before any output is sent.
Output can be sent in the middle of script in some cases. Off the top of my head, I can remember at least 2 such cases:

• f$flush
  There's a test for that case, but for some reason it's not correct
• f$send_http_103_early_hints

Now these cases are skipped, and we invoke callback only in the end of the script. Now it behaves just like register_shutdown_function and it's incorrect

For example:

<?php

function main() {
    header_register_callback(function() {
        fprintf(STDERR, "header_callback invoked\n");
        header("Qwerty: hello");
    });
    fprintf(STDERR, "before flush()\n");
    echo "111111111\n";
    flush();
    echo "222222222\n";
    fprintf(STDERR, "after flush()\n");
}

main();

Expected behaviour here is to have Qwerty: hello response header, but that's not really the case.

Let's do the following:

• find and fix all skipped cases like that
• add test with callback, which sends some RPC to check swapcontext correctness