Remove awaitCloseStatus in SockJS integration tests

A CloseStatus may not be received if the connection is closed while
the client is between XHR polling requests.
Update MockMvcConfigurer support

This is a follow-up on the commit introducing MockMvcConfigurer:
https://github.com/spring-projects/spring-framework/commit/c2b0fac852dd9f865699fb374ad78543767fec05

This commit refines the MockMvcConfigurer contract to use (the new)
ConfigurableMockMvcBuilder hence not requiring downcasting to
AbstractMockMvcBuilder.

The same also no longer passes the "default" RequestBuilder which would
also require a downcast, but rather allows a RequestPostProcessor to be
returned so that a 3rd party framework or application can modify any
property of every performed MockHttpServletRequest.

To make this possible the new SmartRequestBuilder interface separates
request building from request post processing while the new
ConfigurableSmartRequestBuilder allows adding a RequestPostProcessor
to a MockMvcBuilder.

Issue: SPR-11497