Remove usage of `renderToHTML` from all examples

The renderToHTML is a private API that seems to have accidentally leaked into our examples. We should remove the usage of this function from all examples that have it.

Specifically:

• custom-server-hapi
• ssr-caching
• examples/with-flow/flow-typed/next.js.flow

[Charioteer]

@Timer As already stated by @sagar-gavhane there should be an option to manually render a page to HTML. I’ve already commented on @madiknox’s commit that the changes are breaking e. g. the ssr-caching example. The removal of renderToHTML is a critical break and will also break many production apps (like ours) if it won’t be accessible at all anymore. I suggest to add an intended public low-level API (with documentation) for rendering manually. renderToHTML() should not call res.end() as well because it leads to a need of “crazy” workarounds like the following to actually cache HTML responses:

const _resEnd = res.end.bind(res)
    res.end = function (payload) {
    if (res.statusCode === 200) {
        ssrCache.set(key, payload)
    }
    return _resEnd(payload)
}
await app.renderToHTML(req, res, pagePath, queryParams)

[sagar-gavhane]

which alternative public method we’re going to use?