Add StreamingRequestMiddleware to stream incoming requests, mark StreamingServer as internal

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This changeset adds a new StreamingRequestMiddleware to stream incoming requests and marks the existing StreamingServer as internal:

// unchanged: Server class is unchanged
$server = new React\Http\Server($handler);

// old: advanced StreamingServer is now internal only
$server = new React\Http\StreamingServer($handler);

// new: use StreamingRequestMiddleware instead of StreamingServer
$server = new React\Http\Server(array(
     new React\Http\Middleware\StreamingRequestMiddleware(),
     $handler
));

I've tried to keep the changeset as minimal as possible and most changes are in fact documentation only. See also each individual commit. Among others, this allows us to streamline the documentation and will allow us to improve default buffering behavior as discussed in #365/#360 and others. This will be filed as a follow-up PR once this PR is in.

Resolves #343

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@jsor Thanks for spotting, updated