Polish async feature for ServerHttpRequest/Response

ServerHttpAsyncResponseControl wraps a ServetHttpRequest and -Response
pair and allows putting the processing of the request in async mode
so that the response remains open until explicitly closed, either from
the current or from another thread.

ServletServerHttpAsyncResponseControl provides a Serlvet-based
implementation.
Polish ServletServerHttpRequest query param handling

The method returning query parameters now returns only query string
parameters as opposed to any Servlet request parameter.

This commit also adds a ReadOnlyMultiValueMap.
Polish SockJS exception handling and javadoc

See javadoc in SockJsService for details.

Also remove ReadOnlyMultiValueMap, CollectionUtils has a method for
that already.
Remove unintended dependency on Servlet API in SockJS

Add a factory method in ServerHttpRequest for creating a
ServerHttpAsyncRequestControl.