QuickJS: mark nginx wrapper references during GC

QuickJS: mark nginx wrapper references during GC

The HTTP request and stream session wrappers hold JavaScript references
that were invisible to the QuickJS cycle collector.  For example, this
creates a cycle through the native request wrapper:

    function content(r) {
        r.args.request = r;
        r.return(200);
    }

When the context was destroyed, the collector could leave this graph in
the runtime, causing a worker abort in an assertion-enabled build or a
leak otherwise.  Cached request and response bodies had the same
ownership problem.  A stream callback capturing its session formed an
equivalent native cycle.

Add gc_mark callbacks symmetric with the class finalizers.  Stream
callbacks can now rely on normal cycle collection, so remove the special
teardown path that released them before the common exit-hook and pending
job processing.