With gRPC it is possible that a request sending is blocked due to flow
control.  Moreover, further sending might be only allowed once the
backend sees all the data we've already sent.  With such a backend
it is required to clear the TCP_NOPUSH socket option to make sure all
the data we've sent are actually delivered to the backend.

As such, we now clear TCP_NOPUSH in ngx_http_upstream_send_request()
also on NGX_AGAIN if c->write->ready is set.  This fixes a test (which
waits for all the 64k bytes as per initial window before allowing more
bytes) with sendfile enabled when the body was written to a file
in a different context.

Now tcp_nopush on peer connections is disabled if it is disabled on
the client connection, similar to how we handle c->sendfile.  Previously,
tcp_nopush was always used on upstream connections, regardless of
the "tcp_nopush" directive.

We copy input buffers to our buffers, so various flags might be
unexpectedly set in buffers returned by ngx_chain_get_free_buf().

In particular, the b->in_file flag might be set when the body was
written to a file in a different context.  With sendfile enabled this
in turn might result in protocol corruption if such a buffer was reused
for a control frame.

Make sure to clear buffers and set only fields we really need to be set.

The module implements random load-balancing algorithm with optional second
choice.  In the latter case, the best of two servers is chosen, accounting
number of connections and server weight.

Example:

upstream u {
    random [two [least_conn]];

    server 127.0.0.1:8080;
    server 127.0.0.1:8081;
    server 127.0.0.1:8082;
    server 127.0.0.1:8083;
}

With u->conf->preserve_output set the request body file might be used
after the response header is sent, so avoid cleaning it.  (Normally
this is not a problem as u->conf->preserve_output is only set with
r->request_body_no_buffering, but the request body might be already
written to a file in a different context.)

Instead of the connection scheme, use scheme from the original request.
This fixes pushes when SSL is terminated by a proxy server in front of
nginx.

For HTTP/1, it keeps scheme from the absolute form of URI.
For HTTP/2, the :scheme request pseudo-header field value.

The scheme is validated as per RFC 3986, Section 3.1.