Fix buffering output flush, linked to transaction problems

We buffer the HTTP response until after transaction commit time using BufferingHttpServletResponse in order to make sure that a client sending two requests in a row sees consistent state.

In Chemistry the AtomPub and Browser servlets do a response.flushBuffer() in their servlet which currently bypasses BufferingHttpServletResponse. This is incorrect because this may send a result to the client too early. This happens for instance for a 204 (No Content) for which the client won't wait for more.

So BufferingHttpServletResponse must also intercept flushBuffer.